I'm pretty indifferent to Burrow, and probably won't end up with him. However, I'm curious what you mean by the bolded. Burrow led the league in completion % and YPA (which is incredible as those stats are often at odds) and was 3rd in TD%. So the argument for Burrow's upside, is boils down to him having more pass attempts. Which there are 2 reasons to think could happen.
1. An improved OL, leading to both more time to throw (as Burrow certainly loves his deep shots) and likely a reduction in sacks/hits.
2. Being another year removed from his knee injury, he won't be babied/protected as much. He averaged 29 attempts per game in his 1st 6 games. He averaged 35 from then on, which is a pace that would have been top-10 in the NFL.
Again, I'm not saying draft Joe Burrow at all costs, but I can see stats backing up why he could/should be better than he was a year ago, where he finished as QB8.