All you ever wanted to know about thresholds and their history is in the following link.
I suppose it goes without saying that the lactate vs VO2 curves of a couch potato and a fit endurance athletes are very different. The fit rider's curve is shifted to the right and down. In practice, it means the top of their zone 2 will be much closer to CP/FTP/MLSS levels than an unfit individual. Interestingly to me, older marathon racers can do an entire marathon at 90-95% of VO2 max (not a typo).
https://physoc.onlinelibrary.wiley.c....1113/JP279963