Just to add to Delvino's thoughts: I didn't get the impression that Kushner was dissing musical-theater performers at all.
He was praising Jose Andres Rivera and possibly alluding to Rivera's humility because Rivera ''didn't think of himself as an actor.''
Here's Kushner's quote: ''We got so lucky with casting. Josh shocked me after the first couple days of filming. I went up to him and said, 'Josh, you're an absolutely astonishing actor.' He was in the road company of ''Hamilton.'' His training is all musical theater. He didn't think of himself as an actor. But he is an absolutely brilliant actor with an immense heart. He'll go anywhere you ask him to go. ... What Josh does with Chino is phenomenal.''