In my limited experience, the Guatemala aduana people can make records appear, disappear, and change at will on their computers. If some guy said he had no record of your departure, I'd assume he was looking for a bribe. They did the same to me (after 10 months or so), but then punched a couple of keys and made the record appear.
But yes: the rule has long been in place, and their aduana seems to take great delight in paging through passports looking for something to argue about--missing stamps, missing dates, whatever. Actual enforcement of the rules is always variable, but saving receipts and other documentation is a good idea, just in case--same as a lot of places.