The comment defined existence, but didn't define it. The followup commenter was clarifying that proving the is something that exists isn't a proof that a particular claimed thing exists.
"it" is something being referenced - some details in the territory being marked out by the map. The map, and this act, provide no evidence that something exists. The only real basis is if you have evidence there's some details, of the nature claimed by your map, that can have some sort of transitive causal influence upon us.