This comes via reliable dataminer LuigiBlood (delightful name) and reported by VGC. Apparently, the Nintendo Switch Online SNES emulator had always contained the relevant support necessary to implement the SNES Mouse, but the most recent update to NSO significantly updated the data.

Please verify your email address. The September 17 SNES app update expanded SNES Mouse functionality. The potential addition of SNES Mouse games, like Mario Paint, could be in the works. According to backend evidence from a source, the Nintendo Switch ...

According to the user, Switch Online’s SNES emulator – which is used for its library of SNES games for Switch Online subscribers – has secretly contained SNES mouse support since launch. However, what’s notable is that there’s now apparent evidence that it was recently significantly updated in the emulator code.