Calling it obsolete is hyperbole. A standard definition of obsolete is something that was once common but no longer is. Just last week I had to revert a number of ext4 filesystems back to ext3 due to performance problems on RHEL5, so I'm afraid that ext3 will be in common use for years to come.
(The performance problems aren't present in Fedora 16, so you could argue that RHEL5 is obsolete.)
Yes, the RHEL5 kernel is obsolete in that it lacks the barrier support necessary to implement proper behavior with decent performance. The fact that obsolete ext3 runs better on also-obsolete 2.6.18 kernels doesn't really tell us much except that sometimes two pieces of software evolve together, and sometimes versions that were developed together work better than versions that were not.
(The performance problems aren't present in Fedora 16, so you could argue that RHEL5 is obsolete.)