Pretty sure there are lots of valid definitions of "cross-platform", and I will say again that context matters.
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.
With linux desktop use at ~1.5% and dropping, at some point it becomes bad business to support it. It's not adobe's job to pick OS's for their users, it's their job to support the OS's that their users use. And their users are about 99% Windows/Mac it seems.
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.
I mean, yes. Functional cross-platform doesn't mean "every single platform ever", and hitting 98% is a fantastic target.
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.
edit: we are talking about replacements for Adobe products here, so context matters. Adobe products _also_ don't run on Linux.