[MetalGuru]

He’s not saying anyone is trying to argue that slavery is ok. He’s using it as hyperbole that without a specific definition, progress can’t be measured objectively.

[ziddoap]

An appeal to extremes is not the only way to get the point across.

[setr]

Its just an efficient one.

By pulling to the extreme, you can remove a lot of the smaller and subtler points of the argument, to get pointedly at a core issue.

Unless of course the recipient tries to (re-)introduce irrelevant aspects of the fact that you tried to be pointed...

[ziddoap]

I haven't heard of an established logical fallacy being an efficient argument but, sure.