I think I recall reading that around that time (remember this is 2003) Linus was either against -Werror or against spending effort eliminate warnings. The reason being that GCC had a few false positives, and the effort of making Linux kernel build with these spurious errors was not worth the risk of breaking code that likely worked ok.

However I can't find anything where this is directly said, all I can find is a collection of Linus' early 00s emails on the subject of GCC which includes a LOT of reference to said warnings: https://yarchive.net/comp/linux/gcc.html