There is a lot of misconceptions surrounding various thresholds and zones from a metabolic and physiological perspective. The following could explain Seiler's 2 Zone perspective.

We have been conditioned to think we go anaerobic as lactate builds up and it is due to low oxygenation at the working muscles but this is completely wrong. This history of that speculative leap is explained below. There is plenty of O2 all the way to VO2 max, the true limits are metabolics in the working muscles. Or at least that is my current misunderstanding.

https://physoc.onlinelibrary.wiley.c....1113/JP279963