It's neat on a technical level that, considering the emulator emulates Wii U hardware presumably, they are getting it to run better than the Wii U version already does, unless they are fiddling with things to allow it to draw from their whole PCs specs.
Is it piracy? Well I can pop in games I own into my computer and play them with the right programs, its how I keep playing old PS1 games after one of them had a problem with the spinning of my discs. Anyone know the laws on playing software you own on hardware that it wasn't intended for? I imagine it may not be super consistent.
I hope they get it all working and just sit there laughing as Nintendos biggest game for at least most of the year has technical issues on both system releases. Even if minor depending on mode or system that is not a seal of quality for me, get your asses back to the drawing board. Really woulda liked to have BoTW delayed on Switch so it took better advantage of the tech inside.
All in all, emulators can be used for shady purposes or not purely on the player, I'm not gonna harp on the devs of the emulator for anything. If Nintendo has beef I'd donate a few hundred to help with legal costs as they challenge such a claim, you got an issue you talk to players.