I think it happens because this guy don't know about slangs.
The word is about drugs or lazy guys. But it also a slang and the meaning can be "adj. cool, nice, awesome".
Cool also can be: awesome / cool / sweet / wicked / sick / epic / dope / bad ass / tight / gnarly / insane / great / fantastic / incredible / the sh*t / bitchin (slightly vulgar)
"a dope" is a stupid person. This meaning of "dope" is widely known across the world. The person you spoke to may have thought you meant, "you're such a dope" = "you're such an idiot". That's an insult.
I've never heard the word "dope" used as an adjective to be mean "cool" but it may be a modern usage.