In the beginning of the year, during a passing play, Riley Leonard would look for WR1, maybe take a quick look for WR2, and if neither were open, he'd take off running. His short range accuracy was fine, but his medium and long range accuracy were terrible.
In a nutshell, during that time, he was more of a pure spread-option, run-first QB. Opposing teams used a defense similar to the ones that were aimed at stopping a Tommy Rees-style QB (weak arm) to good effect, since they were willing to give up the QB runs.
Squatting in a short zone could limit him to a acceptable gain, especially on 3rd and long.
But, after that debacle against NIU, he started taking more time, and actually start looking more carefully for WR2, and even occasionally looking for WR3 before taking off running during a dedicated passing play.
He wasn't anywhere nearly as good as Ian Book or Jack Coan when it came to reading multiple receivers and throwing an accurate ball to them at medium / medium+ ranges, but because he was no longer just a spread-option runner who could only throw short passes, the "let's stop Tommy Rees" style of defense no longer worked.
Leonard studied a lot of film, spent a good bit of time with Coach Guidugli, and the results speak for themselves.
I think that with good guidance from an established NFL QB coach, there's a reasonable chance he could very well take that next step, especially if he has time to study and digest the NFL game, instead of being thrust into having to play early.
He will never be a precision passer who can read complex receiver progressions an in instantaneous moment (Jim Kelly was one of the best at this), but I think he can be a significantly improved passer to the point where he's a viable NFL quarterback.