Whilst I do agree with your generalisations, I don’t accept that they apply at the volumes being discussed in this thread.
12 hours per week can be structured to be persistently productive for M & F. It is a serious undertaking that should not be taken lightly, but it is achievable and maintainable.
That does not mean that someone (either M/F) can jump off the couch and do 12 hours, but it is straightforward to build up to that volume in a way that makes it sustainable and productive.
A couple of things have caught my eye from your previous posts
“it seems completely unattainable for me without essentially riding Z1 all the time. In particular, any anaerobic efforts result in my muscles requiring an extended period of recovery”
“no matter what I’ve tried. I keep trying to push through this ceiling and end up either burned out or injured. It just feels like my body can’t recover quickly enough no matter what I do.”
“a hard race for example will set me up for a minimum of five days of recovery”
“I seem to make significant gains just doing Z2”
Quite alarming was the “unable to ride at all” part of this “I’ve done 12+ weeks very easy and ended up slower/ unable to ride at all.” What do you mean by that?
Without seeing more data to make a more informed decision, these things point to you doing too much higher intensity work at that volume level than you are ready for.
Maybe your choice is to stick to 6-10 hours and make the most of that time, or do the work (on and off the bike) to build up to the 10-15 hours that you mention in your first post?
Decent cycling fitness can be achieved in both scenarios, but there is no doubt that there is a general link between higher volume and higher fitness in these volume ranges.
Those ranges you have mentioned are wide though: 10 is very, very different to 6, and 15 is very different to 10.