It’s not an exclusive, it’s just not on Playstation or Switch. When did people decide PC releases didn’t count?
Windows is made by Microsoft. Would you feel better if people called Starfield a “Microsoft exclusive”?
Linux runs Starfield with no problems. If it’s on a computer and doesn’t use restrictive DRM to control how and under which circumstances you run the software, it’s not an exclusive. Microsoft doesn’t have exclusives anymore, which is a giant pro-consumer move that doesn’t get enough applause in the gaming community. That doesn’t mean they need to develop stuff for one specific DRM box owned by their biggest competitor to be “anti-exclusive”.
Linux only runs it through a translation layer, Proton, it doesn’t run it natively.
Would you also not call Pokémon an exclusive just because technically it can run on a pc?
Generally, “exclusive” in this context is referring to exclusivity on a console involved in the (IMO completely unnecessary) console wars.
I do agree that PC is an important item there too but, the problems there are a bit different - for example shoddy ports (no justification for porting from x86/amd64 consoles to PC to be bad), excessive and intrusive DRM, and unreasonable delay or unwillingness to port.