I don't think Oliver saw the Havertz/Rodri clash clearly. The shot from behind a goal shows that he turned partially away to make his way towards the Arsenal defence - presumably (and reasonably to me) not expecting there to be anything to watch in the first 2 seconds away from the ball.
I'd also think that the other officials were similar - just not expecting a flashpoint so soon.
I am quite surprised that no close focus footage has appeared of the incident.
There was a league cup final vs Chelsea about 10 years ago when Jorginho (I think) clattered Aguero off the ball in the first 5 seconds - same applied then.