He doesn't need to be better than hall. Hall was good enough (obviously not on the day they ran the olympic marathons) to win.
We know what Rupp is on the track. He is a guy that will finish 3-8th most of the time. We have several WC and a dozen of diamond league meets that demonstrate that. At the marathon we have no clue if he is a 2:04 guy or a 2:09 one. Odds are he is a 2:06 type guy that is still going to be one notch below the top 5 guys.