I think I’m just gonna start posting this whenever this discussion comes up, until it’s more common knowledge what “FTP” was invented to be:
FTP was just a method to estimate “threshold” power, as the thought was to base power zones (when training by power was first invented) off threshold power.
Threshold did not have a precise definition either, but the “threshold” people did by RPE, LT2, MLSS, OBLA, etc were all similar enough that it probably didn’t matter given you don’t need precise values for training purposes (a 5% change is just noise).
40k TT time trial and 2x20 were the methods he originally suggested might be useful to estimate was threshold power roughly was.
Thats it.