who are you, Plato?

It's just a quote. By somebody else. People put such things in their sigs, you know. I give attribution on my homenode.

it does not say in Writeup Formatting Tips to obey XML syntax

Correct. That is because writeups do not need to obey XML syntax. The xml access to a node exists so that other applications can "add value" to PerlMonks. The writeup content is just a single blob within the XML. But you're right, this way (and as far as I know it's the only way) of viewing the real source of another monk's writeup is very far from obvious. I suppose the PerlMonks FAQ ought to say something about this.

Just put if you're not us go away on the monastery gates.

No. But maybe something like "If you're not willing to learn how to get along in this community, go away."