In the world of the show, I’m supposed to think Keith wrote all the songs, right?

Keith was supposed to have written many of the Partridge tunes. This was made clear in episodes like "This is My Song" (S1) and "Waiting for Bolero" (S2). But episodes like "Go Directly to Jail" and "Knight in Shining Armor" (both from S1) showed that the family also accepted material from outside sources.However, since DC was a prolific musician and songwriter IRL, the premise that Keith could be too wasn't that far-fetched.