Your partitioning between those two things is good, but I still don't think that either label applies to them:

> Steam does control the vast share of desktop gaming.

Between the Epic Games Store, GOG, Humble Bundle, Xbox, Origin, Itch, and a few others, I don't believe their control is anywhere close to the fraction needed for Steam to be a "monopoly", either legally or in casual speak.

> Steam does control

...and, what's more, they don't "control" anything - what prevents you from either using multiple clients (on the player side) or selling on multiple storefronts (on the developer side)?

A monopoly has to monopolize some limited resource or market - you can't really have a monopoly if there's no limiting or exclusivity. That's like saying that Fortnite is "monopolizing" the battle royale genre because it's the most popular - it is the most popular, but there's no exclusivity because you can always play another battle royale in addition to Fortnite.

Monopolies need pie charts (limited resources that are taken by a single actor), but Steam is a bar in a bar chart.

I’m using the correct terminology.

Google controls 90% of the search market and the browser market. There is nothing preventing anyone from searching on Bing. Yet, the correct terminology is control of the market.

Google has a monopoly on search. Have they abused that monopoly? That’s a legal question that’s currently in court.

Steams share is somewhere in the 70s and it is far stickier than Google. A gamer can’t abandon their steam library easily. Have they abused this monopoly position? IMO no, but my knowledge is limited.