As far as I know, it’s not entirely about some purism ideal they have in mind - the difference between the two nvidia camps on Linux is the functionality you gain with both drivers, and the proprietary driver is simply more restrictive, so, yeah, I agree that they have a point.
This is the reason I know very well that my next GPU is going to be an AMD one (given that their hardware has proper open source source by that time, that is). I bought by GPU back in 2017 or 2018, I think, a couple of years before using Linux and even considering it - had I known that today’s me was going to run LInux, I would’ve gone for an AMD GPU right away.
Even skipping the Nvidia driver debates, the AMD hardware has been a much more consistent and pleasant experience for me on Linux overall across several AMD-based laptops that I have installed Linux on. While I did manage to get things going on my desktop that has an Nvidia GPU, it definitely caused me more headache than I expected.
I think the easiest counter-argument here is healthy disagreement.
Being exposed to multiple opinions is undoubtedly important and is far, far better for us all in the long run than only limiting ourselves to only those opinions and views we already share or at least like, but having an option to wall somebody off on an Internet platform has its benefits, too, like not actually wasting your time in endless and fruitless arguments. As great as it would for everyone to be able to have a healthy and productive conversation about the differences in their views, it simply isn’t wise to honestly expect that from everyone.
Besides, having two opposing ideas communicate on the same platform is not what the fediverse is for - not exclusively for sure. It’s the freedom to self-host and self-regulate places dedicated to specific things to various degrees: lemmy.world, for instance, is wide and large and encompasses many things at once, and has an option to federate and communicate with smaller, more niche communities and vise versa, while letting the users open a single account with either.
Otherwise it’s just the old Facebook formula of encouraging opposing views to constantly clash for the sake of engagement. That’s just not real, not healthy, and only exists for the purpose of being some sort of KPI in a corporation perpetually hungry for money and influence. So yeah, we don’t want that.