@arnan You appear to be arguing against a definition of "stack" which is clearly not the one being used in the original post.
"Stack" is not being used as a synonym for "set of languages a person knows". I think it's probably a stretch to think that it's being used as a synonym for "set of languages a system is written in" rather than "the system".
@sgf it's broadly used for stuff they use/know. Eg. Languages, frameworks, maybe even a working routine. Which is different for everyone, so what is a full stack? Yours is probably different than mine. Hence it's a pointless term.