I think it is more to do with the console remembering the highest game version. I've done it a bunch of times too if a dlc or update corrupted my games, but only certain games have given me the warning about needing an update in order to play even if I repacked the game with no fw or key requirements. (still talking about game update btw, not fw updates)
Sometimes deleting the full game from the switch settings and then reinstalling the base game did the trick for me, but then applying the update again just to uninstall it in devmenu to test it, gave me the same error. (Can't remember if I saw this on 3.x, but I had it on 6.x and now 7.x, if they did change something in the firmware to detect updates it's probably been done in 4.x or something and not just the latest 8.x)
That's why I don't think it's to do with the firmware specifically, maybe to help prevent save game corruption, like how some games wont let you use a save on an earlier version of a game. Or because ther's no way for you to naturally remove an update without deleting the whole game on ofw.
It's possible that the switch assumes you have a missing update due to a corrupted download etc, and forces you to "repair" the game by reinstalling the update.
These are all just ideas I thought when I got that message, I don't know for sure if any of what I've said is right.
