I posted this in another thread, but it makes more sense to post here.
If you still happen to be looking for this and don't want to deal with that file sharing site, I can confirm that it's on Usenet. There are a few obviously fake releases, but I was able to find the legit one by searching the file name prefix "matt-xbdc" in nzbking (you can probably find it on other indexers, I'm not a big Usenet guy and just happened to find it there).
There's also a large collection (that appears to be distinct from the MATTRiCK collection though there is some overlap) in that big leak "HDD.torrent" that's been floating around, then another smaller collection on Archive org under the name "xbox-360-xenon-recovery-xdk-setup". Between all of those, it's a fairly complete collection of 360 XDKs (of course it'll never be fully complete, but it's a huge number of build numbers in total) and they're all still pretty easy to find once you know what to look for
