It is a common misconception that antitrust violations require a monopoly or something close to it. Some antitrust violations only apply to actors with large market share, some don't.

Although this is situation is likely not illegal for other reasons

I would assume that it’s like the Chrome browser does not allow you downloading Firefox using it, surely that would be illegal, wouldn’t it?

https://www.justice.gov/atr/antitrust-laws-and-you