Thats really easy for Reddit to measure. Why are you assuming they haven't?
It strikes me as a very shortsighted move.
If you stop assuming that Reddit is run by idiots, and you consider the likely probability that they've modelled this stuff in some depth, it's easy to believe that your initial assumption is wrong, and that the users are on 1st party apps (or will be if others shut down), and that many will stay and continue to post rather than leave or stop posting.
Your premise is based wholly on the belief that you know more about Reddit users than Reddit does. That seems dubious to me.
I know that their UI and native application are absolutely bullshit.
It's a well know historical fact that companies always do the smart move only based on hard facts.
Something that so many founders get wrong is the belief that something needs to be good to be valuable. It doesn't. It just needs to be better than not having it. That is often a tremendously low bar.