According to Deadline, he blew everyone out of the water with his audition:
"...the circumstances behind his landing the starring role are exceptional. He got the lead after his first-ever screen audition, immediately winning over the filmmakers and execs, particularly Barris. Harlow’s got game too, as evidenced by his play during the recent NBA All-Star Weekend’s Celebrity Game. The filmmakers now are looking to set his costar as the movie is on a fast track."