I think this dissonance comes from most of YC's guiding literature being written by PG in the 2000s and early 2010s. Back in those days it was definitely possible to grab the attention of users and investors with janky prototypes. But now the reality is that prototypes are nothing and traction is king. The new YC partners probably have to make this shift in practice and out of politeness, they don't call out the guiding literature