He means it as a metaphor for "they'll say you're extremely evil," and Mengele is the most evil scientist that's also a household name. Treating any name like it's Voldemort isn't a helpful norm for a society to uphold.
I'm pretty sure Scott is Jewish, so I imagine he's aware of Mengele's work.
Blame the Twitterati for the normalization of calling people "Nazis" over slight political disagreements.
It's a Twitter thing now, but it might be useful to understand that it predates Twitter.
E.g., "Goodwin's Law" is supposed to have been coined in 1990, and of course it was a pretty well established phenomenon to earn a coined term.
I know from first-hand experience it was happening on BBS's and newsgroups in the '80s. I would be more surprised than not if it wasn't happening before then too.
(And if you want to generalize slightly, calling someone a Nazi is simply a current way of demonizing your opponent. People have been demonizing their opponents since well before there were Nazis.)
I think we ought to re-invigorate "Philistines" as the term for "people who disagree with us even though we haven't listened to what they said". It's got a better flavor, ya'know? and the whole historical angle should appeal to the kiddies.
Usenet wants a word.
Well, as a German I feel obligated to recommend those officials to read up on Josef Mengele and then reconsider throwing that name around so casually.