It's a ridiculous argument. The Nazis went through a LOT of effort and resources to gather Jews from all the corners of Europe, and even more effort into exterminating them as fast as they could, within the logistical and economic constraints of fighting a 3 front war.
There's no comparison at all to the ease with which Israel could just drop a couple of bombs on Gaza, had it decided to do so.
The fact that I just spent five minutes thinking about it proves that it's not ridiculous at all. The scale is different (so far), but I’m not convinced there’s a qualitative difference.
The only thing stopping Israel from doing that is international outrage. Israel is entirely dependent on its benefactor states like the US and, while it pushes the limits to the extreme, must at least contend with world opinion.