Art song is a genre. It includes works by Monteverdi, Dowland, Purcell, Haydn, Schubert, Wolf, Schoenberg, Messiaen, Tippett, Kurtag... it's not defined by any technical feature but by being part of that tradition.

I might be more inclined to think someone had something to say about it if they could spell composers' names right and drop the bigoted crud about performers' dress code.