If you look at the signees (especially the two or three that actually seem the most invested in this message, and responded to the discussion thread linked above), you will see that these are not the people building "core infrastructure" used by "millions". It's a random grab-bag of default Gnome applications, the kind most Linux users who are aware of alternatives immediately remove from distributions that ship it. "Random Gnome developers" as a group have been notorious for decades for not really gelling with the wider Linux ecosystem.

Looking at the project maintained by the front-runner of this movement, it has had 5 total reported issues in the last year, none of which are related to theming, so I'm not sure I understand what the big support struggle they're chafing at is either.