why do Vagrant docs suggest using public IP address 33.33.33.10 for local VMs?

The vagrant documentation does not appear to suggest to use 33.33.33.10.

Digging deeper into the official documentation, I was only able to find RFC 1918 addresses in use.

Given the apparent close association between Vagrant and this incorrectly-used address, I can only surmise that the docs did refer to it in the past, and were later corrected.

Oh, and you definitely should not use someone else's public IP address, even on your own private network. Very Bad Things can (and probably will) happen.

As to why they've suggested you do it I'm going to go with "they're idiots". But really that's something we can't answer here. Why do people say or do anything? You'd need to ask the person who wrote that documentation.

As to what you should do, you're absolutely right: You shouldn't be using public IP addresses you don't own.