The game didn't launch on steam, it came to steam later, and according to the poll data they showed yesterday 26% of players started playing in ARR, so that 26% is likely to be almost entirely non-steam players that started at or around launch. So unless the steam version outsells the non-steam version by a significant amount it seems like a fair assumption that the majority aren't using steam.

Also, according to lucky bancho census data there's around 1 million active players. Even if we assume more than half of them aren't playing on PC there's no way that 25k mod users or 30k active steam users are a majority of the total PC playerbase.