They screened all of these people, did they not?

Yep - they weren't showing up at the southern border with no ID, that's for sure.

You're once again assuming that all terrorists are the same James Bond type. They're not.
Some of them are the planners who know the details.
Others are just the trigger-pullers.

Funny, I think you have it backwards.

The trigger-pullers are the ones who should just fly in. That's not a "brains" job, and it takes a fair amount of brains to be able to bluff your way through multiple interviews with DHS officials with a completely fake cover story that makes zero sense. The U.S. Border Patrol encountered exactly 34 Yemenis at the Southern Border in 2019 (out of close to a million encounters) - if you're one of them, you're going to get an extra look, which makes you especially likely to get caught (or locked up) if you aren't really good at spycraft. And if you have no documentation at all, it's just going to be harder.

If you have a fake passport, you don't have to do anything other than answer three or four questions at a crowded customs counter. If you try to seek asylum at the border, you're in for days of background checks and several one-on-one interviews starting off as one of the most unusual things to hit that sector in months. It's idiotic to send a "trigger-puller" into the latter situation rather than the former.

cbp.gov: PDF document