I do not have an english screenshot on hand but no joke, they wrote instructions on how to use the link feature for every game where you can do that for both GB and GBA NSO. Mario Kart Super Circuit, which is already on the GBA service, is one of these.
In fact, NSO has instructions on the link feature for every single game on both the Game Boy and GBA NSO apps. Update : After some more digging, LuigiBlood has found even more evidence to support link cable features on Nintendo Switch Online.
