I don't understand this mentality. If we oppose monopolistic sales platforms when it's Amazon, Google Play, or the Apple store why should we turn a blind eye when suddenly we like a particular company.
I'm not contesting that Steam offers the best user experience by a mile (it truly beats Epic and Gog by miles), but that doesn't erase the downsides of having a single entity with a grip on the entire market.
It's only double counted in a situation where you're actually counting both sides. This is a Canadian study published by a Canadian outlet about the impacts of Canadian policy.
They're not trying to balance the books, so to speak, they're evaluating transactions on a single account.