I mean, they develop and maintain Proton yet they don't even prohibit you from using it on other things. If literally any company did that, their shareholders would riot...
...so I don't think it technically qualifies as a monopoly, but they probably could have had a legal monopoly using an exclusivity patent on the tech (although they technically can't patent the whole thing because it's based on Wine, but they could have done this in a way that they could have).