So the only explanation is that it's a good idea because, well, Stevens cannot be stupid?
Who said it was a good idea? The word was "smart". Stay on target.
Available evidence suggests that the three-point shooting prowess of these young players could go either way. Neither player is comfortably league-average over a large sample, but at the same time neither of them is unspeakably atrocious, and both of them are young.
How about the obvious fact that it's a bad decision, much like starting Bradley at PG for a good part of the season? Or did that not happen as well?
Nothing obvious or factual about that.
Bradley has had an offseason to grow, and he's still young. The prospect of him turning into a halfway decent PG (not even a good one) is something worth pursuing. If at any time AB can run the offense without laying an egg, that gives a lot of potential match-up advantages to this team, in terms of lineups (we can play big, and have strong defense).
Maybe it never happens, but it would be a boon to the team if it did. Stevens clearly has the mandate to experiment.