They are probably referring to the fact that in the Linux world, in-tree modules get a lot of maintenance for free when the kernel refactors the pieces beneath.

If you have ever written a kernel module out of tree then tried to upgrade the kernel version you know this pain.

There are no QA processes per se for all file systems. The kernel is pretty much a bundling of many projects and the big file systems are their own projects, managed largely indepently from the Linux masters.