I touched on this in another post on this topic.
The offences that lead to a direct free kick or penalty are when a player charges, jumps at, kicks or attempts to kick, pushes, strikes or attempts to strike (including head-butt), tackles or challenges, trips or attempts to trip an opponent in a manner considered by the referee to be careless, reckless or using excessive force.
In addition to these offences, a direct free kick can be awarded for the following offences which don't need to be careless, reckless or with excessive force: handball, holding an opponent, impeding an opponent with contact, biting, spitting at someone, throwing an object at the ball or an opponent or a match official, or making contact with the ball with a held object. (Law 12).
A push isn't specifically mentioned, but a player can be penalised for a push, and this is covered by the offence of impeding an opponent with contact. So for the Salah penalty, the referee must have been satisfied that the contact by Dias against Salah actually impeded Salah.
There was clearly contact between the two players, initiated firstly by Salah, and secondly by Dias, so we need to ask whether or not either contact impeded (delayed, halted, prevented) their opponent.
I think we can agree that Salah didn't impede Dias. Did the contact initiated by Dias impede Salah? My view is that is didn't. It did not slow him down in any way. Salah overhit the ball, and realised he could not score. That wasn't a consequence of the Dias contact.
Ask whether that contact could possibly have caused that reaction (legs buckling, arms being thrown in the air, a cry of anguish, and falling to the floor) and the answer is clearly no. Put these things together and you have minimal physical contact leading to an exaggerated, implausible and unnatural reaction. Therefore, it wasn't a penalty, and Salah should have been cautioned for using unsporting behaviour under law 12 (simulation - attempts to deceive the referee, e.g. by feigning injury or pretending to have been fouled).
I don't see how logically, under the written Laws of the Game, that Liverpool should have been awarded a penalty for that incident.