M0 baseline difficulty and item lvl is the equivalent of S03 M10. So the difficulty, ilvl jump and other factors of what existed from 0-10 are now baseline on Mythic zero. M+2 = S03 M11 and so forth.
I think the inclusion of “0” is a mistake, when you look at the graphic for the difficulty compression they previously provided in mid-March. Clearly the before and after for M0 is NOT timed in that image. However, it’s clustered in difficulty with timed keys, with the intent that a non-timed M0 will be on par with the difficulty for a +2 key, but M0 will not be “irrelevant” after a few weeks… the difficulty being matched to S3 for a +10 key.