I also think it's finally sunk in that the wall idea will never actually work. And then someone said "iron dome", and his brain concluded that a dome sounds more bad-ass than a wall, anyway.

Recall he has said that his temperment now and when he was in first grade is "not all that different". So whenever you try to work out his thought process, it helps to think like a seven-year-old. (Not six: If Trump were a kid today, his parents would have definitely held him back a year)

Doesn't dementia affect a person's ability to recognize metaphor?

Maybe he just watched Biodome with Pauly Shore?

If you watch the surrounding context, he is absolutely talking about a missile intercept program. He referred to "MIT graduates" going "bing bong boom" to make an incoming missile "completely gone."

So all the comments acting as if he wants a physical giant dome are attacking a strawman while the actual fact he's taking about missile defense like a five year old is lampoon-worthy enough.