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.