Weird to see a DLC for a game that's older than Steam. Would have been more interesting if they released the WADs or whatever that you could play on the open source version of the game. (All of this is interesting in light of MS's recent hallowing of their game studios.)
When Quake was popular, CDs were far and away the most popular install method. They're locking a lot of OG owners out (unless they're hoping to drive a trickle of $4 Steam sales).