While I don’t disagree with taking some electrolytes, the issue is overdrinking, not water vs. sports drink. Unless there’s new data I’m not aware of.
And likely the culprit is SIADH, not merely drinking to excess. Probably lots of people who drink way too much and never have an issue.
Just yesterday in the “Why do experts always disagree” thread we had comments on this very from Dr. Tim Noakes. I am reasonably sure that Dr. Noakes is one of the authors of the very guidelines you quoted to us here and he has been on a big push to eliminate hyponatremia in races.
Pertinent to what you have said he points that it is overdrinking that causes the hyponatremia not the presence or absence of electrolytes in the liquids. That the hyponatremia is a matter of dilution. There’s been some basic research showing this apparently and he also pointed to the death of someone in an event who was drinking gatorade.
If you didn’t catch the thread, and it seems like you didn’t since you were working at the race yesterday; I’d suggest giving it a look.
Nicely said … there have been many prior threads on this topic on ST. Taking in electrolytes isn’t the problem with those who develop hyponatremia … this is a different issue than just being dehydrated. IM with finishers up to 17 hours of exertion versus marathon alone at 4 hours +/_ probably is a much bigger exacerbator of both hyponatremia and dehydration issues. Sticking with medical literature facts lets people make better decisions 
Dave