M's nature is the same as Sharnoth, a realm which lacks absolutely anything, no sound, no color, no time etc. That's evidence he has some sort of causality. In addition we have multiple statements of time being meaningless to him, and he predates creation itself, having always existed as the the "Black", and obviously as Sharnoth which is a part of him, has no time, the same can likely be said about causality, that it's meaningless to him.
In addition his nature is stated to be beyond the time, space and causality of several infinite series of universes.
@RM97 Can probably explains this better, since he is the one arguing for it.
Personally i am interested in the other aspects of M's nature, reposted what i said previously
"M's nature is a fantasy, pun intended, on the one hand he is a a concept which makes up the world, while also transcending it, and on the other he is the king of a race of non-existent beings, said race were dreamt up by humans, and went extinct due to the light of civilization, but not M, cause obviously he has existed long before humans or even creation itself.
He is a concept, but said concept apparently doesn't exist? Darkness you would think exists, but Sharnoth which is an extension of him doesn't, but Sharnoth is said to be infinite darkness, so does that mean that darkness doesn't exist? I mean Sharnoth is said to have nothing, yet everything. Nobody is there in Sharnoth yet everybody is there.
Then there is the whole thing of Green Stones which can affect nonexistent beings, not being able to affect him."
Sharnoth a part of him, has nothing, yet has everything, nobody is in Sharnoth, yet everybody is there. M is stated to be incomparable to the Old Ones, who are type 1 non-existent beings, and in addition Green Stones which can affect non-existent beings, don't affect him, though you can probably chalk that up to him being a type 2 concept i guess.