Why it's not MS is simple. Kakashi never mentioned it as MS & Madara's eye never transformed. Had the space/time jutsu been a MS dojutsu it would of been mentioned in the manga already.
Madara as a young adult/teen 399-01
Madara meeting Itachi with longer hair & a slightly different mask 400-8
. Please be aware that at this time Obito was already dead. His hair style is completely different from Obito here. There is quite simply no way that Madara would be able to transfer to Obito's body after this metting sense Obito was already dead.
Madara's face shown to be that of an old man with wrinkles. 396-17
, & 397-03
Old people in Naruto have similar wrinkles 398-4
Obito is dead & his body was compleatly crushed by rocks. 243-18
, & 244-10