On the other hand, if you count downloads of all the 3rd party Reddit apps on Google Play, it'll sum up to at least 10m. So 3rd party apps users might actually be in range of 10%-20% of users, which is far from negligible
That's only on Android, on iOS it's likely another 100 million or likely more (Reddit has around 500 million to 1.5 billion monthly active users, the vast majority of which are mobile, based on their stats). Apollo is around 10 million, so 20 million total / 200 million is 10%, which is high but not that high compared to overall.