Yeah, but Id expect the sentence above ("Sei solo?"), where "solo" is clearly an adjective, not to appear in the 'Adverbs' section. I'm surprised they don't distinguish between the different uses of the same string of letters "solo". That's what I'm saying, not that "solo" can't also be used as an adverb.
I know, but that's the way Duolingo teaches it to us. It likes to keep new words that we learn together in the same skill, even though one usage is as an adjective and another as an adverb. That way, you don't encounter «solo» 3 skills later and translate it as "only" when it means "lonely." Then people will get confused and say that before «solo» was taught as "only" and now it means "lonely."