Being a marketplace for other people's games would probably be classified as a different type of income, specially if steam sells software that's not games (which it does).
I'll grant that a looot of valve's revenue is from marketplace & steam store, and sure they sell other products but it's a drop in the bucket vs gaming. It's effectively a service for gamers. It's probably not considered gaming revenue for accounting/internal politics reasons.
I would wager that if it were possible to, you'd be able to link a vast majority of valve's revenue to the purpose of gaming. Reality vs financial reality?
According to Wikipedia, Steam made over 3 billion in 2017, but they also earn from microtransactions primarily in Dota 2 and CS:GO, which continue to be some of the most played games on Steam.
But how could Valve have made $6.5b off of their own games? They haven't released anything in yonks have they?! Plus EA have the gacha-fication of FIFA/every other sports game franchise they own with Ultimate Team, etc.