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.