The joke is in the text, you just have to read it. However the text wrongly paraphrases it, but the offense is the same.
He basically belittled a good chunk of minorities that use the #notyourshield hashtag in conjunction with gamergate.
It was a pretty lame joke. We should really be more offended that he was allowed to make a "how many X to do Y" joke on a stage.
If you demand respect or gratitude for your volunteer work, you're doing volunteering wrong.