Oh Yahtzee, you scamp!
Not surprising for two reasons:
One, Yahtzee goes faster with his gags than the old cocaine-fueled Robin Williams and he does it by being incredibly wide-ranging.
Two, antivax people are reviled in large sections of the Internet world. Even aside from the websites created specifically to go against them, the majority of the userbase on Reddit, Imgur, Slashdot, and most large social media websites thinks the whole antivax idea is idiotic and worthy of mockery.
The key word there is large: Small echo chambers can go 'round the bend on any number of issues, but once you’ve attracted enough people from the surrounding community, the views within your group begin to reflect the views of the surrounding community more and more, and the underlying Internet community, as sampled by these large-userbase websites, is provax. (IOW, regression to the mean works.)
(Reddit is particularly prone to this. It’s divided into fora known as ‘subreddits’ and each subreddit is practically its own board, with its own community, subject to the regression to the mean I described above. There are antivax subreddits, but they’re small and not representative of the mainstream.)