To be really precise it was about measuring the size and distribution of all sorts of skull irregularities (the proverbial 'bumps') and mapping them to various traits, it's basically palm reading for the head.

Siskind is just being his usual disingenuous self, i.e. 'everyone always uses skull shape' (to indicate that my intellectual precursors were clowns) is obviously referencing phrenology, then immediately motte-and-baiieys it to a claim of correlation of cranial capacity and IQ.

Except for M&B sleight of hand to work the claim shift shouldn't happen in the same sentence, otherwise it's extremely obvious that you are claiming one thing while carrying water for the other thing (phrenology), which is probably why he ended up deleting the post.