My understanding is that for now the developer chose to declare the newest Windows OS because that's what they're working on and therefore can safely say the game works in every release.
(1) Linux users had previously reported that various older builds of the game worked on Linux and there have even been recent reports of the current build working
(2) Goldhawk Interactive was planning to eventually officially ensure that X2 will work on Linux.
So right now Linux is not officially supported but should be in the future, and meanwhile the game might
run on it anyway.
Thank you for the hope. Since Linux will be supported in the future I'll wait and buy the game in the future :)