That's exactly what I was thinking. There's no way at his age that his max is below 180 and it's most likely in the 190 range, which, like you said, is under 80% of his maxHR.
So yes, that is pretty crazy impressive..... though, why do a 20 mile endurance run as a "test" run? Wouldn't an 82-84% of maxHR be better as it would better mimic the fueling requirements of a marathon while not dipping into the AT hr's of 85-87% where he would need to be efficient at using lactate as fuel (see Canova) and risk breaking himself down? Maybe he stuck with an 'endurance' effort purely for strength reasons and AlSal is leaving marathon specific workouts for later in Rupp's career....?
Thing is, if 4:52 pace is in the 75-78% of his maxHR range, then AlSale is being conservative saying "Rupp can do 2:05 someday."
I for one, though, trust in Kara Goucher's accusations and don't give in to the misogynistic perception of her associated with her taking a stand; that same misogynistic perception is what discredited Betsy Andreu for so long. Her voice was stifled when she had been telling the truth the entire time. I equate Kara's stance as parallel to Betsy's.