Reading through all this I think a point needs to be made. While testing may have been down, they will still be being tested at the Tour and many of these riders have years of passport data logged. It's certainly not perfect, but it would be a big risk to go thermonuclear and risk the protracted legal fight that would no doubt come about if they tripped the system. While the risk is decreased, it's still a big risk and you know that the backlash would be massive as people can see the opportunity to dope given by COVID is large, so assumptions will be very strong.
As for hypoxic training via the use of masks, it's not something I've read up on hugely but from what I recall the science is pretty much up in the air (which a lot of the time means it doesn't really work). I remember Matt Hughes and Pat Miletich wearing gas masks in a video I saw when I was into MMA, and I think Marshawn Lynch was linked to trying them (which seems weird for something that you'd think is for endurance athletes) but that's it off the top of my head.
I think there's just too many variables to draw any conclusions. Some riders will have been able to train outside, some won't. Some will have had their team around, some won't. Lockdown has been tough mentally on everyone and pro riders facing months away from their family are no different. Some will cope better than others. Cycling is also a very "traditional" sport in how it is done. The dogmatic view is that you need racing in your legs to perform at your best and this has always been how schedules are developed. The amount of racing you need is varied, and you might not be going all out at every event, but that's how it's always been. Maybe now they're finding that actually very little to no racing would be better. Or maybe it's better for people who really can go deep in training, but others need the competition to achieve those last few % points of their optimum level. Anecdotally, when the local bunches and chain gangs started up a couple of months ago, the speeds were all stepped up. There were more riders out, which will have an effect, but KOMs and records were getting broken by reasonable margins and there wasn't an increase in the category or riders out, it was still the same folk who are out every year, plus some newer racers of a similar level.
Essentially, I think it's fun and interesting to speculate, but unless several people get popped or this situation goes on for several years, it's going to be really hard to unpick all of the changes that have happened and work out which ones might have made a difference, if there really is a difference.
*Edits because lockdown has destroyed my ability to use coherent grammar...