It’s because they compared the US Direct footage and ROTW footage that they realised we got the 50Hz ones:
During the North American version of the Nintendo Direct broadcast, the games shown ran at full speed. However, when compared to the Nintendo UK edition of the broadcast, the footage was noticeably slower, which hints that the games European players will get will be 50Hz, rather than 60Hz.
Also, Nintendo made Super Metroid, in 50Hz, run on 60Hz output on the Wii U.
This is Nintendo, remember.