The last part is "the message" of a scientific publication (the "why I worked on/wrote this").
Why was his memo unscientific? 1. His choice of data presentation was almost exclusively for his side of the hypothesis, of the form "women are biologically inclined to X" without including any of the (vast corpus of) counter-studies* that directly offset many of his core supporting arguments.
2. Even if his arguments were sufficiently sound (which is unlikely as per 1.), his conclusions were a bit of a quantum leap from his "sourced" claims to "this is unfair, divisive, bad for business" (as per the TLDR). He pretty much just decided that for himself. There were few or no studies/data/observations to support that crucial step.
*going to piggyback off the sources linked in this slate article: http://www.slate.com/blogs/xx_factor/2017/08/07/the_google_a...