Trademarks have to be enforced or they can be lost, so it makes sense to be overbroad about them. You say you could have fought it but that doesn’t mean you were legally in the right.

In this case, everything on their site is legal and above board.

Admittedly, Nintendo doesn’t care if what you’re doing is legal if it could cut into sales of current systems, games, or merchandise - they’ll issue takedowns regardless. That’s why videos of people demoing the MIG Switch got taken down for copyright infringement, for example. But given that every system this can extract games from already has its entire library available online in the form of pirated ROMs, getting it taken down won’t do anything for their bottom line.

In fact, Nintendo taking legal action against products like this would encourage piracy of their games. If a consumer wants a backup of their physical game cartridge library and the tools to create such backups are made unavailable or harder to access due to Nintendo’s actions, that consumer is likely to simply download the ROMs instead. That’s already piracy, and it’s only a few clicks more for the user to download ROMs for games they don’t own (and if you’re already legally a pirate, that line in the sand is awfully faint). And sites that host ROMs for the Gameboy Advance probably host ROMs for newer systems, too - including the ones that Nintendo actually cares about - so it’s in Nintendo’s best interest not to push those consumers in their direction.