I'm not sure how Apple/Google 30% relates? Those companies do a lot to support businesses on their platform. If you don't want to change anything, you get free hosting. For payments, they provide the entire infrastructure to support developers. It's a mutually beneficial arrangement even if the fees are quite high.

If Reddit does want to kill all third party apps complaining about how they haven't done a single thing to actually support 3rd party developers is kind of moot. But the point of the protest is how underhanded that is; they would be manipulating users and developers by lying about the whole thing. And if they actually didn't want to kill all third party apps then they still lied and are also incompetent.