This usage of "a couple" is so common that it's far beyond some weird thing some people say, and telling a non-native speaker it's wrong is not doing them any favor - it's basically universally accepted usage. FWIW, if you have a reference to a styleguide that explicitly addresses it I'd be interested - I did a short search and didn't find anything, but I also don't have easy access to the full text of some major ones. But even then, disagreeing with some style guide doesn't make anyone "illiterate", or "needing english lessons" (which by the way explicitly teach people that meaning of couple, because you need it to understand English in practice).
(and I didn't flag your original post, because I don't think merely being wrong justifies flagging)
> idioms for couple
a couple of, more than two, but not many, of; a small number of; a few:
It will take a couple of days for the package to get there.
A dinner party, whether for a couple of old friends or eight new acquaintances, takes nearly the same amount of effort.
And some more dictionaries that agree:
https://www.merriam-webster.com/dictionary/couple
> an indefinite small number: few a couple of days ago
https://www.oxfordlearnersdictionaries.com/definition/englis...
> couple (of somebody/something) a small number of people or things: We went there a couple of years ago.*