“The inventor, Coggan, used a 40k TT as one way to measure it, a 20-minute FTP test as another.”
No, he didn’t…and therein lies the rub.
“Newer studies show that only a small minority of cyclists can hold their FTP (measured with the 20-minute FTP test) for an hour, including pros.”
What these studies actually show is that 95% of 20 min power (usually w/o the 5 min “blowout” effort beforehand, thus not even following Hunter’s protocol) often overestimates maximal metabolic steady state power, a.k.a., FTP.
“Untrained individuals will fare much worse on average. That always reminds me of a video where a fitness Youtuber had his then-untrained wife do a VO2max test in a lab. She never came close to VO2max.”
Untrained individuals are no less likely to achieve a true VO2max during an incremental exercise test than an elite athlete. It simply comes down to motivation and an appropriate test protocol.
You are correct, though, it is always good experimental design to have people practice a performance test one (or more) times before relying on the data, as there are often learning effects. Yet, very few, if any, of the various studies of “FTP” ( i.e., 95% of maximal 20 min power) have done so. This is disappointing, but not all that surprising, because the quality of much applied sports science research is rather poor, reflecting the rather uncompetitive nature of the field.