I'll admit the same thought crossed my mind when I saw the winning time.
But here is what would likely have had happened:
Hall WOULD have followed for the first 1/2, and blown up like the others.
When you say he likes to run his "own race", that has been somewhat true at Boston, but even then he's gone out hard (with the lead pace), died a bit, and then finished strong. But it was never enough to come close to winning. So I am pretty sure that that would not have worked today if he tried to hang with TODAY's pace (tougher than he's ever done).
You also forget (maybe?) that when he ran his PR at london, he actually DID go out at an insane pace (sub 1:02, correct? 1:01:4x...?). So he didn't run his "own race." In that race, he died a bit, and hung for a great time. He would have run sub 2:06 that day if he had run an even race, but he didn't. So likely , an "in form", or "back in the day" Hall would have done the same thing, as in gone out with the leaders, but at an even harder pace, and died, but probably harder. (I am assuming weather was worse today than during Hall's 2:06 london race? if so, more reason he would have died).
so yes, if Hall ran a completely even split race, and was in peak form, he would have indeed had a shot. But I highly doubt a peak Hall would give the leaders 1:30 at the half, since in the past he actually would always go with the pace, no matter how fast, through the 1/2 or at least near the 1/2.