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.
Zeus and Sherlock Holmes could both be argued to have had transitive causal influences upon us - so in order to make that statement make sense there must be some other implied constraints on what is considered to "exist."
It all depends on what you mean when you say "The fictional character Sherlock Holmes". I believe that thoughts, memories and knowledge "about Sherlock Holmes" exist and are physical details, as are Sherlock Holmes books, etc. And that there's no other reality to "Sherlock Holmes", above and beyond these things.
But really, this discussion is too large to satisfactorily deal with here.