edit: we are talking about replacements for Adobe products here, so context matters. Adobe products _also_ don't run on Linux.
If I'm talking about a mobile app and I say "cross-platform" I probably meant iOS and Android. It would be strange to say "what about Linux??" in that context.
So here we are talking about competitors to Adobe products, where "cross-platform" means Windows and Mac, so when I say "cross-platform" in this context, I am also referring to that same set of platforms.
When you cover about 98-99% of your users platforms, I think you're cross platform.
Otherwise, we could define a cross-platform slippery slope such that no software has ever achieved it. There's always another platform you didn't support.
Does it run on more than one platform? If yes, it's cross platform.
Otherwise, where do you draw the line? Linux? BSD? Haiku? Minix?
Listen, I love linux, and it's not MY fault that the year of the Linux Desktop never came and that its marketshare has continued to fall against its competitors.
The intersection of artists, who are heavy users of the Adobe suite of products, and linux aficionados is essentially zero.
You do realize this, do you not?