Ifyou take this one statement out of context, it might seem so. I think she is distinguishing between the cases 1 and 2b in the kwiziq lesson.

(1) J'aime ton pull. Il est très joli.
(2) J'aime ton pull. C'est un très joli pull.

Both sentences use an adjective but the first one talks about a specific sweater. You might think that the second sentence also talks about the same specific sweater, but it does so in a general sense indicated by the indefinite article un.