That bias graph is made assuming you are picking numbers between 0 and 32768. With a 64-bit hash the bias is negligible. With a 32-bit hash I suppose you could argue significance around 100 million items.
I think the text is clear that the reason the hash table supports power of two size tables is to avoid the modulo operation.
I think the text is clear that the reason the hash table supports power of two size tables is to avoid the modulo operation.