I feel stupid because I thought the "love theme" was played during the credits.
Don't feel stupid. "The Love Theme" is the instrumental music played over the end credits. It has never had a vocal track.
The Love Theme is played over the opening credits. Unless you can think of a good reason why somebody's credited for Lyrics on an instrumental.
Technically, it's also played over the end credits, since it's just the same tune slowed down.
In television and cinema, the "Love Theme" version is the instrumental one. (As in, "The Love Theme from ________"). This is what they were riffing on, so the "Love Theme" must be the outro, at least logically.
Admittedly, I never noticed before, and sure enough, it gives "lyrics" credits to Joel and Josh.