Threshold power is the highest intensity that does not eventually lead to achieving VO2max. Agree?
No. It is defined as the power you can hold steady for just an hour. VO2max ist by definition the max oxygen consumption your body can achieve in ml/min (or ml/min/kg) – when perfectly fresh.
Take e.g. your 30min max power threshold, so slightly above FTP. When you fall off your bike after these 30min with 30-min-power you will NOT have achieved VO2max, even so you generated constantly more power than TP.
Then what is the cause of fatigue when doing 99% of threshold power vs. 101% of threshold power?
Threshold is defined as when lactate clearance cannot keep up with lactate production.
This can be at anything between 40 and 70 minutes
There are different threshold definitions across FTP, Critical Power, MLSS, (2nd) lactate threshold, (2nd) ventilatory threshold (and plenty of arguments between people that know a lot more about it than me on other forums). And in practice it varies day to day (and maybe even during the day) so I don’t get too hung up on the nearest watt.
Xert definition is here for info. If you check your one-hour power on the modelled Xert curve, you’ll find that it’s actually below TP. My Xert TP seems to be a bit (but not massively) above what other tools would suggest for an estimated FTP, and I’m nearly certain I couldn’t sustain it for an hour, no matter how fresh or motivated. But it doesn’t bother me as Xert still seems to predict when I’ll fail with above TP efforts pretty well!