What does Acts 21:38 mean?

Paul is buffeted amidst false charges, slanderous accusations, and mistaken identity. Legalists falsely claim he teaches Jewish Christians they do not need to be circumcised (Acts 21:20–21). Through a circuitous route, this leads to the accusation that he has brought a Gentile into the temple (Acts 21:28). When a Roman officer arrives to break up the ensuing riot, he somehow assumes Paul is an Egyptian who recently led peasants into an ill-fated revolution.

In Cyprus (Acts 13:8) and Ephesus (Acts 19:13–14), Paul encountered Jewish frauds who provided supernatural diversions for the religiously unfulfilled Gentiles. In Judea and Samaria, the people were preyed upon by charlatans who promised significance of some kind. In AD 36, an imposter, who claimed to be Moses reincarnated, convinced a group of Samaritans that if they excavated Mount Gerizim, they would find gold artifacts left by Moses. Pilate, misunderstanding the gathering, killed many in battle, took others prisoner, and executed the leaders. When the survivors protested, Pilate was removed from his post.

Josephus, in Jewish Antiquities, 20.8.6, recorded an event that occurred three years before this riot at the temple. An Egyptian came to Judea and told people if they joined him at the Mount of Olives, he would destroy the walls around Jerusalem, and they could rebel against the occupying Roman forces. Felix, the governor, found out and sent horsemen and footmen who attacked the would-be revolutionaries. They slew 400 and took 200 alive, but the ringleader disappeared.

It is this Egyptian with whom the tribune confuses Paul. "Assassins" refers to the Sicarii, extremist Judaean nationalists known for assassinating their political enemies in crowds with small, curved daggers.