Soo... Can't we use the Pikmin1/2 ROM plus custom injection for playing any GC games, like in a real Dolphin emu?
Well, for one, it's based on the Wii versions, so even if it was as simple as swapping out the ROM file the answer would be no
But, like the 3D All Stars version of Mario Galaxy, there
is no ROM; it's a port. The emulator (something Nintendo's European R&D subsidiary wrote themselves; unrelated to the unofficial Dolphin Emulator) is just for graphics and audio. The code directly targets Switch and was compiled natively for it.
And we know from their release of Mario Sunshine (a game that was essentially fully emulated) that their solution to make it perform well is basically to do a lot of the emulation ahead of time and pre-translate all of the game's code to the Switch architecture - a process we can't feasibly replicate. Pikmin is a particularly light game so had there been no Wii version, and they chose to go the emulation route instead of porting, they may have tried dynamic recompilation, and you could inject arbitrary GameCube ROMs. But even then, most games you'd try to inject would be slow to the point of unplayability (the reason they did static recompilation for Sunshine in the first place).
They'll almost certainly be using this emulator on future Nintendo consoles, ones powerful enough for them to release games using a dynamic recompiler that runs a wide selection of games at an acceptable speed. But the Switch just isn't there, unfortunately.