"Mesa is our most important upstream and KWin (together with Compiz and Mutter) are your most important downstreams." is not even close to be true. They're very important, but they're not even close to be the most important. Reality is that no one is getting paid to make sure KWin works on latest Mesa3D and that by itself makes it pretty insignificant. In fact there's very few people running KDE as their main DE at all here.