According to Microsoft almost everything from Microsoft is cross platform, because it works in both Windows 10 and 11. That's good enough for Microsoft to be cross platform. I'm not even kidding!
It's a movie reference. Specifically it references "The Lord of the Rings: The return of the King" at the end of the battle of Minas Tirith.
It is said by Gimli, as a response to Legolas slaying a Mumak (giant war elephant).
I mean they can require it if they want. Apple would just bury the fact that it exists and would use something that's known to be shit. Idk how they could be forced to make iMessage cross platform.