from the article,
"HRV has been shown to be a predictor of illness in elite athletes, but its ability to predict injury is yet to be validated in humans."

Hath not a human hands, organs, dimensions, senses, affections, passions; fed with the same food, hurt with the same weapons, subject to the same diseases, healed by the same means, warmed and cooled by the same winter and summer as an elite athlete is ? If you prick us, do we not bleed ?

To me that article seems to have way too much irrelevant detail obscuring the real questions.

A better overview is at Outside,
https://www.outsideonline.com/...ke-me-better-athlete

I suspect the articles that show HRV is not a useful indicator of training state, are based on HRV apps with proprietary algorithms analyzing the data and producing a simple 'score'. Using your own raw data and tracking it over time to build a baseline, is going to be more difficult but more likely to be useful.

Start here for research showing HRV considered useful,
https://www.ncbi.nlm.nih.gov/pubmed/11039645

In general, to Slowman's original question - my take is that anyone promoting a single metric or score to measure peak fitness/overtraining is guaranteed to be wrong at some level, and most often misleadingly wrong. They are also probably trying to sell you something..