Should be, theoretically. But in fact - it is not =)) In fact it is ALWAYS a lot of work to adopt build to a new platform and support updates, include a lot of testing at least. Especially if game contains a lot of complicated content and platform-specific code, include installers, updaters, Direct-X version-specific shaders etc. Also, not every Unity plugins works on Linux properly from the box, like video players for example. All this stuff is need to be managed before we can say "we support Linux".