In context, they were talking about the specific case of a source that outputs 33% 1s and 67% 0s, with no correlation between successive outputs. For the genera

In context, they were talking about the specific case of a source that outputs 33% 1s and 67% 0s, with no correlation between successive outputs. For the general case of a entropy source that's (not provably not) biased in some arbitary way that you don't know about, you obviously need a cryptographically secure hash function.