I would be surprised if that line in the prompt caused it without the other thing they did just before mecha Hitler: Elon created a Twitter thread asking users to submit divisive politically incorrect facts for grok training. It was full of Holocaust denial and white supremecy stuff.

https://x.com/elonmusk/status/1936493967320953090

Most likely they rolled back finetuning on that thread at the same time they adjusted the prompt.