It is not at all like starting a heli with the blades not fully unfolded, the Mavic shakes a little for only a fraction of a second. An unbalanced heli can shake violently for several seconds while spinning up, with the possibility of bending the main shaft or other problems.
While I agree that a folded spin-up can't be good for the Mavic, that does not mean that it is harmful to it. You could do some math and calculate the actual forces on the bearings, then compare that to the ratings of some generic similar sized bearings. I think you will find that the forces don't come close to anything that would cause problems.
Yes, failing to unfold the props will cause additional stress. That is certain, but the additional stress is insignificant. Actually flying the Mavic for a couple seconds will cause more wear and tear.