If I recall correctly, Microsoft didn't adapt the Linux kernel. As far as I have studied so far, they seem to have studied the base specifications that allow file system mounting and program execution (perhaps this has something to do with Unix and the Linux Standard Base?), and nothing else. Leaving Canonical to provide the rest of the packages from the non-[free/libre] system distribution called GNU+Linux Ubuntu (including the GNU packages provided there).
Now, you're right when saying that we must still consider Windows 10 as non-free, specially because it doesn't teach users about their essential freedoms, see: <https://mikegerwitz.com/2016/04/GNU-kWindows> (important note: this post **might not** represent the views of the FSF).
