Nope. They document what words are in common use. English is a "form follows usage" kind of language, where popularity of a word makes it correct. That's why "literally" can mean its own antonym and influencers get to make up new meanings for Fetch and Mid.

Less architectural, more suicide note.

Partly right, but they don't decide if a word is "official" (whatever that's supposed to mean). For a word to be a so-called "real" word it only has to be in common use among some group, dictionaries simply document words that have been in common use. Merriam-Webster is an authoritative record of words in use specifically in US English (with some records for other English variants and dialects, I think? ) but they are not a prescriptivist organisation. A word which appears in their dictionary is almost certainly a word that is or was in use in US English but a word that doesn't appear might also be a real word, particularly if it's a relatively new word or meaning.

So with that in mind, arguing that a word is real when it doesn't appear in the dictionary can be valid in some cases, but arguing that a word isn't real when it does appear in a dictionary (like Brian did) is generally not smart.

tl;dr, a dictionary, not the dictionary; not all English; "official" doesn't make sense here; in some (but not this) cases disagreeing is valid.