You do seem to have been super-vociferous on this and keep stoking it. Have you a dog in this fight?
However, "the important thing here" are the post-testing facts.
It seems that tests suggest it was not e-coli from the water caused sickness before MTR but Norovirus (person-to-person) - ITV report:
"Norovirus is the "most likely" explanation for 88 athletes becoming unwell after the World Triathlon Championship series racing in Sunderland, the UK Health Security Agency North East said."
UK HSA:"After the event, 88 people contacted the North East Health Protection Team to report experiencing diarrhoea and/or vomiting symptoms after the event. 31 faecal samples were then sent to be tested for a range of viral and bacterial pathogens.
" 19 showed evidence of Norovirus infection while the remaining samples either tested negative or were positive for other infections including sapovirus, astrovirus and rotavirus.
"E.coli was present in four samples but not the kind associated with severe illness. It can be carried naturally in the gut of healthy individuals, so it is not possible to say whether the presence is a result of participation in the event.
“Norovirus is a very unpleasant stomach bug but tends to pass after a few days with most people usually making a complete recovery without any specific treatment. Norovirus can easily spread from person to person, particularly in large groups."
https://www.itv.com/...land-world-triathlon

. . . about water quality.
On race-day(s), maybe the water was safe and maybe it wasn’t. Has anyone seen or know of a test result from the day(s) of competition?

EDIT to update: I just received an email (sent to all participants) from British Triathlon acknowledging that some athletes reported sickness (which is now being investigated by relevant authorities) and that the test on Wed 26th showed high levels of e.coli. They say the test on the 26th was not taken in the harbour area where the swims took place (but instead on the other side of the pier) … hmmm. More clearly though, they shared the results of 2 tests done on Sun 30th (AG race + MTR). The e.coli levels in those samples were 10/100ml and 45/100ml - so a safe level. No mention of the water quality on Sat 29th when the individual elite and junior races were held.

I'm hoping that there are knowledgeable people on here that can help us understand this. Completely lay person here, but keen that any lessons are identified and measures (need to be practical) be put in hand to make any changes necessary. No changey; no learney.

Here's the water test report for Roker Beach on two relevant dates: "TRI/BriTri tests were carried out".
Tests were to determine E. coli Enterococci and pH levels.

https://www.triathlon.org/...tement%2BMatrix1.pdf

[edited] "health problem due to contaminated swimming water is associated with microbiological contamination. . .
. . . most important is human and animal faecal contamination: "associated pathogens: enterococci/faecal streptococci, E.Coli, faecal coliformi, total coliform, salmonella, shigella, campylobacter, vibrio cholera, gastro enteritis viruses, enteroviruses, parasitic protozoa, some other exotic parasites, Aeromonas and Plesiomonas."
The two bold ones are tested for, assume as markers of contamination to allow judgement of water quality.
"Faecal contamination can harbour many potentially pathogens. It is not feasible to test all those micro organisms. Further on there is also a lack of data considering the quantitative relationship of the concentration of many of those pathogens and the risks of disease. Especially the viruses are the most hazardous."
"Commonly used indicator
Bacterial indicators of faecal contamination considered are enterococci/faecal streptococci,
E.Coli, faecal coliformi and total coliform. Thermotolerant coliform bacteria are present in
human and animal faeces. However they also occur in other environments that bear no
relationship to faecal contamination (discharge from paper industry, brewery’s etc).
Escherichia coli (E.Coli) is a species of faecal coliform that is specific for faecal material from
humans and other warm-blooded animals. Enterococci are a subgroup within the faecal
streptococcus group and are distinguished by their ability to survive in salt water. Faecal
coliforms as a group were determined to be a poor indicator of the risk of GI illness (US EPA
2002)."
"Bacterial and or viral tests
. . the microbiological quality of waters has been measured by the analysis of indicator microorganisms. Human enteric virus are the most likely pathogens responsible for waterborne diseases from recreational water, but detection
methods are complex and costly for routine monitoring and so the main parameters analysed for compliance with the Directive are indicator organisms.
EEA Standards
The Directive 2006/7/EC reduced the number of parameters from 19 to 2 key
microbiological parameters and the faecal contamination is assessed by
determination of two mandatory indicator bacteria:
• Escherichia Coli
• Enterococci
The choice of microbiological parameters is based on available scientific evidence provided by epidemiological studies conducted by the WHO and health Institutes in Germany, France and Netherlands.

Is the current TRI water quality test protocol fit for purpose? (idk)

https://www.britishtriathlon.org/...-report-07_12_23.pdf
"Open water swimming is known to be associated with risk of GI illness with outbreaks reported in many countries involving a range of pathogens. There is well-publicised advice about reducing the risk of illness, focusing on general hygiene measures both during (don’t put head under water, don’t swallow water) and after swimming (wash hands / shower asap after swimming, . . . ) but the challenge in . . . triathlons where the swimming is the first part of the event, is that athletes [have to put head in water, are likely to involuntarily swallow water and are unlikely to take time during the competition to [wash hands or shower]"
I also picked up / deduced (and this relevant to an event which is individual one day and relay a day later) that if there's any doubt about water quality, maybe better to pass on the swim familiarisation, or at least be super careful and take the hygiene measures described to above.