Switch supports hardware texture compression, possibly all of the largest assets have been massively compressed in switch version.
What are you talking about? ALL modern hardware since 2001 supports texture compression. Even.the gamecube and Wii. Some supports more and better formats than others tho.
I have been thinking about this issue yesterday and there is actually one feasible solution they might have used: put the best LOD for models and textures on the patch. In this way people can still play the game (albeit in lower asset quality) without the patch, and experience the best image quality with the patch.