The 80% isn't super surprising considering that he shot 77.7% on FTs last year. It still remains to be seen if that carries over to his 3 point shooting, but given his good shooting from the corners I think it might in the next couple of years (he won't be a 40% shooter, but 37% or 38% isn't out of the question)
And to add one thing about end of quarter shots: according to basketball reference, he has only taken 8, but they said the same thing before the game against Minnesota. He attempted a heave at the end of the 1st quarter of that game, but it didn't register as a heave, so take their numbers with a grain of salt