Are you sure he was talking about this test? In this test I think the second keyboard sounds more reverb-y than the first one. He's posted more than one blind test on YouTube, and in one of them (also between Yamaha and Casio, but different models) I did think the first keyboard sounded more reverb-y.
Okay, I see where he said "P.S. Sorry, one piano has reverb EFX more than another." But I don't think he meant that the first one has more reverb, rather that one of the two (he doesn't specify which one) has more reverb. Based on my own observations of comparing my Casio CTK-710's Grand Piano sound to my Yamaha PSR-E433's Grand Piano sound, I think I know which is which in the blind test, and one of the two brands (I won't say which) does usually have a lot of reverb turned on by default-- so he may have meant that in the default settings for one of the two there's more reverb than in the default settings for the other one.