I just tried this with a Spotify song and Siri said "sorry, I'm unable to share that". It probably does a song search like you said and it failed this time.
I can see a use case for this, though: when you play Apple's radio stations in Music.app (and maybe other stations with rich metadata?), the "now playing" area pretends to be the current song. In that case Siri's implementation would send a link of the current song instead of just Apple Music Hits.
I can see a use case for this, though: when you play Apple's radio stations in Music.app (and maybe other stations with rich metadata?), the "now playing" area pretends to be the current song. In that case Siri's implementation would send a link of the current song instead of just Apple Music Hits.