In what was is human diversity increasing? Broadcast television has been causing the decline of regional accents and increasing cultural homogenization for decades. Many obscure languages are dying out in favor of consolidation at the regional or national level.

In the past, humans were naturally diverse simply because there were so many distinct communities that hardly ever interacted with each other. Nowadays you might be exposed to many times more diversity than the average medieval peasant ever was but there's nothing contradictory about that happening as the total amount of diversity in the world continues to decrease.