The no begging rule is in a Beginners and Help thread that is entitled "NEWBIE README" however there is a header above the "no begging" rule entitled "forum rules" which implies such rules apply to the entire forum. Additionally, this post by theymos says that he "tends to agree" with the forum having a "no begging" policy, and here TheBear says that he will only bite the hands off threads that (among other things) are garbage like begging. Just because the "Unofficial list of official rules contains a weak or an incorrect reference supporting a rule does not mean that rule is invalid.
The "no begging" rule is also somewhat of a "common sense" rule, while all of the marketplace specific rules are not.
I would say most importantly, it appears that one moderator (who happens to be new and less experienced) is interpreting a rule in a different way then how other moderators are interpreting the same rule. A report should be handled the same way regardless of which moderator handles such reports (assuming they do not ignore the report), as it would not be fair to the person making such post if their post was deleted for no reason other then the fact that Cyrus handled a particular report while another person whose circumstances are identical does not have their post deleted for no reason other then the fact that grue handled the report.
I am not the person who argued that the post in the OP was deleted for arbitrary reasons, and I do not think that, I think that it was deleted because a moderator interpreted a rule incorrectly. With that being said, I do not believe that moderators should cite the rule that moderators can use their own interpretation of a rule when backing up a decision. After a (very quick) search for this rule, I was not able to find where a moderator actually stated that this is a rule, however I was able to find this quote by -ck:
So it seems that the ability of a moderator to interpret the rules is a way to automatically close any loopholes in the rules, and not a way to give moderators unlimited discretion in their moderation. I believe that moderators should cite rules, precedent and facts when backing up decisions, and not "rule 23"