Going as far as boycotting Rogan because he's not being critical enough of his guests seems pretty dumb to me. It's a mainstream podcast, and he has high profile guests. No wonder he's not going to grill them.
As to the paper, I think you can poke a lot of holes in it. For example, AFAICT they found that hate speech went down, measured via the frequency of words commonly used within these subreddits. Certainly much of the decline is then attributable to the subreddit's subculture getting killed, and taking with it their slang words. It might just be that other words are in fashion now. And it's not a given that this even generalizes from deplatforming subreddits to deplatforming individuals.
So I still think it's a stretch to call it substantial research, but without getting hung up in any more details, I think the fundamental effectiveness issue with deplatforming is that by taking away the platform, you don't change people's opinions. You just won't see them talk about it anymore, and that's you as in you, the one who banned them, and no one else. They'll still be out there. It's just that now, you're sticking your fingers in your ears and going lalala (this is also mentioned in the paper). See the democrats losing to Trump in 2016 even though basically the whole mainstream media demonized him and his supporters.