I think its more based on economics. If Metro Exodus sells poorly on the PC then he believes that it would mean Metro 4 (or whatever) is then less likely to have a PC version. The lost profit arguement you use only works if the game actually turns a profit on the platform. If a potential boycott causes a loss then that's what they have to go off.
Honestly, I really don't get the fuss about Metro not being on Steam. Epic will be creamed if they didn't invest in their store when they're up against a competitor as dominant as Steam. Microsoft did the same in the early days of the 360 to try to syphon off some of the PlayStation base.
While economically I can see what you are saying, it would be them willingly ignoring the context of why people are boycotting in the first place though and acting like it's the consumers doing it for no reason or that the games don't sell on PC or something, not in response to decisions they made. Especially when prior Metro games have both sold significantly well on PC already, according to SteamSpy both Metro 2033 Redux and Last Light Redux sold between 2-5 million each, I don't know if that includes the original versions in that number either.
So in this case it's basically more like blackmail, or at least sounds like it. "Don't like it? well you better still buy it or else we won't even put them on PC anymore".