Steam meets the definition of a monopoly which is above 50% market share.
https://www.merriam-webster.com/dictionary/monopoly
I see "exclusive" in the definition, I don't see "50% + 1".
Not to mention being sued as a monopoly in several lawsuits
I could sue you for running a monopoly. I won't be successful because it isn't true, but I could still bring it to a court. Charges being filled doesn't mean they are true. That's why this is a court case and not a judgment.