I heard back from Ironman and it is a 16:30 total time cutoff based on chip timing. Announced about 2 weeks before the race. Needless to say I'm not very happy and will spend my $ elsewhere in the future. Good luck to everyone coming to Indiana to race this Saturday :)

Just because there've been different answers to the cut-off question, from page 12 of the guide:

The run course will close 16.5 hours after the last athlete enters the water. Each athlete will have 16.5 hours to complete the entire course. Any athlete that takes longer than 16.5 hours to complete the entire course will receive a DNF.

Someone posted in a facebook group on Wednesday that it was 69.8 at 730am.