It's really no big deal. Dean always had good ideas and a great sense of humor. The reason I was curious is because I just recently realized that this was written during the Jan & Arnie phase. When you really listen, it sounds like a Jan & Arnie song(except for Dean's vocal of course) Listening to that 8 minute video really shows how talented Jan was very early on. For people to discredit his writing ability shows their ignorance of all the early songs he wrote before Surf City.
I guess Jan wanted to include Dean as writer in the beginning so it would be like he and Arnie. I'm sure Dean shared ideas and Jan credited him. But eventually, Jan attached himself to songwriters like Don, Artie, Brian, Roger, etc. Jan always gave credit to songwriters. He was very generous with that. Maybe to the point of diminishing his own songwriting legacy. But I digress lol