There's no way this poll is accurate. Just looking at the Play Store the official app has 100M+ downloads. The bext closest, rif, has 5M+. I think people underestimate how many casual users go straight to the official app and don't care about any of this is.
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.
Keep in mind that rif has been around since 2010, and the official app launched in 2016. I think the opposite is true, anecdotally I've noticed discussion of 3PA has gone down in the past 5 years. People used to talk about it all the time, I didn't hear much until this controversy. Most redditors are not power-users, I don't think many of them would know what F-Droid is let alone use it. I'd personally guess that 3PA usage is under 10%.
I think this change was mainly sparked by cracking down on AI scraping bots, and killing 3PA was a two-birds-one-stone side effect to prepare for their IPO.