UPD: on another page it is explained that the map is just released a bit ahead of schedule for donors. I'm still not convinced about that, but I confirm it's definitely much better this way at least. Quoting:
> ... release of SuperTuxKart was accompanied by a gift package containing new tracks, that would later make their way in the main game.
https://blog.supertuxkart.net/2025/10/supertuxkart-15-release.html