It is true, but you do need some extra items. Not only the Gameboy Advance, but a Gamecube, a GBA controller cable, the Gamecube controller adaptor (the one made to support Smash Bros. games) and a Gameboy Player with boot disk. But if you have all of these items, none of them need to be modded. It’s all Nintendo code and hardware, baby!
The process is to boot the Gamecube and Gameboy Player with the GBA plugged into it with the cable. Then turn off the Gamecube leaving the Gameboy Advance on, disconnect the GBA cable from the GC, then plug it into the Gamecube adapter plugged into a USB port connected to the Switch. If the GBA is still powered on, it should be usable as a Switch controller at that point, until it’s turned off!
If you don’t have them all that kit, a modded GC or Wii will also suffice for the Gameboy Player. It’s all demonstrated in NotRealEric’s video. There is also some alternate hardware usable, including third-party adapters and connectors. The video is ten minutes long, but the setup is in the first 3½ minutes of it; the rest is demonstrating some use cases, including Mario Kart 8 Deluxe.
The hardest to find of these devices now, if you don’t use modded hardware, is the Gameboy Player, which is quite expensive used. But if you do happen to have all the pieces, it’s quite the hack. It seems to work due to a controller-emulation program the Gameboy Player (or modded GB or Wii running mGBA) sends to the GBA, which, amazingly, is Switch compatible.
Maybe you have a magic combination of gizmos lying around to use this trick, and use a GBA as an extremely limited Switch peripheral? Yeah, let’s not kid ourselves, this is pretty silly and doesn’t have a strong use case, considering that Switches come with two Joycons, but it’s amazing that it works at all.
