Perhaps he did "co-write" them, but "co-wrote" is a very loose term, it can imply any degree of involvement in the creative process depending on who says it. Arguably, laying a bassline down in a song could contribute towards making it yours, or you could say "unless you did 50% or more of the work, it's not yours." (a very hard measure to make with these things and very hard to prove). It seems to me that while Matt's work with the rentals was no doubt influenced a lot by his work in Weezer, the differences are marked enough so that I'd doubt he deserves flat out "writing" credit on any Weezer song. And despite my suspicion and my previous post, no, I don't know what went on in the writing process of early weezer songs, but it seems to me that the burden of Weezer suckitude rests squarely on Rivers. I don't think it really makes a lick of difference who's playing bass with them.