Kolasinac's arm raises to balance his body as his leg swings in the air. Natural movement. Ndidi didn't (or did Ndidi? Maybe Ndidi didn't) change momentum or direction so irrelevant. His arm was moving down until he realised Kolasinac was about to cross then started raising it.
If Ndidi didn't change momentum or direction then he has a every odd running style with his legs so far in front of his body and both bent at the knees
In summary. Both Ozil's and Ndidi's respective handling of the ball could have been given as an offence. But they weren't. In the context of the game, it was probably about right, because if one had been given and the other not, then there would have been a disparity.