They offer help now (to now just words and words are cheap) right?
They can retract it tomorrow, or drop the work half done or after two years, just saying "This do not fit our business plan, so bye bye".
Mir compatibility is a downstream problem so apply the patches downstream and enjoy.
About kubuntu, I think that for them will be more easy to maintein a graphical stack based on wayland rather than maintain dowstream patches for import Mir compatibility in Kwin
All the linux ecosystem will test (and help in bug fix) the graphical stack based on wayland and because wayland adoption is upstream, KDE will take care of any bug report about kwin-wayland, so it will be a sharable workload between all distros, KDE/Gnome/Others devs and Wayland devs, instead of all the effort on the shoulders of the kubuntu's team.
Different story to maintain the patches Kwin-Mir downstream, made for the needs of kubuntu only.