MK's lore is no more canon than anyone else's. It is fan-fiction, nothing more. Bethesda put some of his work in game as a sign of respect and a nod to him but that only makes the stuff they include canon. The rest of his writings are far from canon and people that say it is don't know what canon is. Head-canon is not the same as real canon. As wikipedia states(bolded is my text):

"Canon is the material accepted as part of the story in an individual fictional universe. It is often contrasted with, or used as the basis for, works of fan fiction[MK is now just a fan since he is no longer a developer]. Fan-fiction would be described as 'non-canon' while an event from the official source[the TES games] material is 'canon'."

If wikipedia is not good enough for you, let's look at the definitions of canon. Merriam-Webster's 3-B definition is "the authentic works of a writer". In this case, the writers are the CURRENT, not former, Bethesda writing team. Their works are displayed in the games. The stuff that MK writes are fan-made stories. Fan-fiction. Tons of threads devolve into arguments about canon but there is really none to be had. Fan-fiction and head-canon are not real canon and MK's works are not more canon than any other fan's except for the work of his that has been included in TES games.

Did you know that it was Michael Kirkbride who created the White-Gold Concordat, the Void Nights, and gave Kuhlmann the idea of reinstating the Thalmor?

The games explicitly say that canon doesn't exist. It's all just whatever you want it to be. From Divining the Elder Scrolls:

"The prophet who reads the scroll sees one version of what might be. A different prophet might have a different vision with equal veracity."

That's why the title of the series is The Elder Scrolls.