well not to split hairs with you but name me one piece of gaming software that actually follows that "standard" my previously mentioned Apple OSX and Apple iOS are application level software and they don't even follow that "standard"
I get that. For (more or less) self contained software apps, the app makers can call it whatever version they want because no other piece of software depends on it.
However, in the world of software components, plugins, DLC, libraries, and so on, SemVer is really useful. It prevents a "breaking change" in Software_A from messing up dependencies in Software_B. With SemVer, Software_B can require any version "5.x" of Software_A and it will still build fine.
Anyway, when Stardock wants to call it "2.0", that signals to me "this is a big update and all your 1.x.x DLC/plugins/mods won't be compatible with this new version."
For the ordinary non-software-developer, it doesn't mean anything anyway, so it's not a big deal.