Because his argument is that even adjusting for the pipeline issue, women and minorities are under-represented. In other words, even if you fixed the pipeline issue, they would still be discriminated against and under-represented because of bias.
I don't even know what this means:
"0% of partners at some of our largest Bay Area VC firms are women"
So > 0% of partners of the largest Bay Area VC firms are women, since he qualifies the statement with "some"?