Everything is about context and this will more often than not determine a refs decision.
The critical aspect is that Dias makes a mistake it appears he tries to flick the ball away from Salah instead of clearing his lines. Salah intercepts and is in on goal. The ref see this and Dias reaching out appearing to pull Salah back.Salah throws himself forward as usual and hits the turf. The ref makes a decision and in the context of the above not an unreasonable one.
It is the job of VAR to dig deeper into this if it is believed that the ref made his mind up incorrectly. The argument weak in my opinion that Dias was fouled is outside of the context of the main event which in the refs mind read defender fucks up and attempts to interfere with forwards run on goal.
The lesson to be learnt from all this is simply one for Dias when in a spot of bother clear your lines. Salah proved yet again as we have known for a long time that he is a diving, cheating prick.