if the developer released it as a windows build but is being played though a compatibility layer, yes. Unity and Unreal both support deploying Linux builds, but it doesn't mean making a proper Linux port is as easy as pressing the "Linux" button.
>Where are we drawing the line here?
I don't personally care for what counts or not. I just personally wish for more native support.
Depends ... It can be that easy, sometimes. I was maintaining a huge Unity based VR setup the last years which had both Linux and Windows PCs (mostly for legacy reasons). Building for both platforms was done from the same bash script with the only difference being the platform identifier.
Tbf that was a very standalone application that did not interact with the OS a lot, but otoh I would assume that a lot of games are like that.
As someone who has played a lot of games with native linux "support", I want less of it.
In nearly every instance of these native ports, switching to using the windows version via Proton was a better experience, either because the Linux version was outdated, unmaintained and buggy, or it simply performed better.
Annoyingly, as far as I can tell, Steam these days doesn't make a distinction between native ports and Proton games so it's hard to tell if I'm getting served the unloved child version until something goes drastically wrong and I have to start messing around with it.
Any kind of port can be of high or low quality, though.
Sure, I can see that. My solution to that one day will hopefully be to make sure devs can keep their linux platforms updated, not give up and go around it with a windows build.
But Proton discourages that, not encourages. As you said, Steam doesn't want you to know what build you are playing, and if the audience doesn't know, then the devs won't care either.
At least that's my very limited experience with linux gaming. I started only a couple of weeks ago, but so far, everything works incredibly well.
In the mid '10s Feral (and a to a lesser extent Aspyr), ported many AAA games to linx. Many of the ports were of quite good quality. I own a few of them, yet occasionally I have to switch to the Proton version as the native one fails to start.
OSS games are the exception of course: being able to produce a good working binary from source make them future proof.