Interesting question. Something missing from a lot of these study summaries is a measure of the quality of the high intensity work. My experience has been this goes up because you are fully recovered.

I think a big limiter for cycling is the burden put on a few muscle groups to get the necessary aerobic work in for the whole body (vs say XC Skiing). Getting in the volume w/o breaking down your legs is a real balancing act & I think polarizing gives you more margin for error.

Guess I'm saying I think both, don't think they can be separated.