If it can be used as an adjective, then, strictly speaking, it is an adjective.
It might have not been an adjective at older stages of the language, and emerged as a reinterpretation of an older noun, but it is an adjective now – since it is used as such by millions of native speakers.
What is “correct grammar”? Who decides on it? Native speakers use it as an adjective, it is an adjective.
And, besides, dictionaries like online Cambridge dictionary list is as an adjective – even if some do mark it as informal (but Duolingo doesn’t stick to formal register of English so why anybody care even if some speakers wouldn’t accept it in contexts requiring it?).