I mean, I also thought there is probably not that many people... BUT your comment made me actually think otherwise. RIF's 5% is actually huge!! And that is just one app; we don't even know how many are using FOSS apps through F-droid... so I'd say the third-party app ecosystem is probably very, very big, I'd say easily at least 30 % of mobile users are third-parties.
How many millions of users actually installed the official app and went "oh, god, nope", or found out about better apps later on... I'd say that is also pretty big. And not happening the other way around.
So, pure speculation here, but this is probably Reddit's desperate move to change a very possibly growing tendency of third-party app users.