That's not the context of the GP's statement. They're referring to the Universal License being offered by Serif, which provides you a "buy once use anywhere" on all of their supported platforms, not that they support all platforms.

As a Linux user myself and a former (and upcoming again) studio sysadmin, it's disappointing Serif doesn't offer native Linux support. But let's not twist the discussion here.

All supported platforms, as in you don't have to pay a separate license fee to run it on one of the other platforms they support.

Yeah, I'm not happy having to keep a copy of Windows for graphics software either, but that's a different issue. If you only have Linux or some toaster running NetBSD, then it's pretty simple: don't buy it.