Well, the Dolphin emulator was an open source project that was developed while the Wii was still on store shelves and they failed to react. Not much they can do provided it is proper reverse engineering of the console without the use of Nintendo code.
If anything it is the CEMU emulator is what people need to worry about, as it has developed rapidly as a closed source project and is effectively selling new builds on a Patreon that earns over $16,000 per month. It is a much easier target for Nintendo if they wanted to try anything legal.