Mahomes had *barely* crossed the plain... It's unreasonable to expect a defender who is in a dead sprint, desperately trying to get a player out of bounds when every single yard gained is vital, to know exactly when he is out.
I initially thought that it was too close of a call to make in that situation, but upon replay, Mahomes had one foot almost off the chalk (even further out of bounds) when the shove came- touch, then shove. Ossai's arm isn't extended with the shove until Mahomes is off the chalk.
When a QB is running out of bounds and giving himself up, NFL guys know not to even touch them at that point. Even Ossai admitted he has to keep his hands off the QB in that situation and his teammates weren't up in arms about the call. Some were frustrated with Ossai and some supported him in a tough time.