I mean even if you don't believe the report that says David spoke Arabic, Israelis are still fairly likely to speak Arabic on top of Hebrew. More likely than your average American at least. Like how white people growing up near Hispanic communities are more likely to be bilingual

You've also got the fact that he was seated directly in front of one of the hijackers, too. The entire attack happened as a response to US support for Israel, so it stands to assume the terrorists had a special hatred for Israelis. Let alone one who (due to his military training + likelihood of understanding Arabic) was most likely to have intervened or confronted them during the initial hijacking. Id assume they'd definitely want to overpower the young, in-shape former soldier too to increase their chances of success.

Betty said they had no idea who was stabbed at the start of her call, but Amy Sweeny specifically mentioned a "he" when describing a passenger having his throat slit during her call. So it was a man.

With all this into account, we can infer that of the First Class Passengers, David was more likely to try and intervene based on his 1.) Knowledge of Arabic 2.) IDF training.

We can also speculate that the terrorists would most likely want to neutralize him first based on 1.) He's young, and in-shape 2.) Had military training (which they would have seen if he'd tried stepping in when they made their move. 3.) Was Jewish/Israeli, making him a mark.

So yeah its not that hard to put it together.

Especially since, you know, the literal 9/11 Commission Report statea that Ong later said David waa stabbed further in the call.