I don't know if Jefferson will be a first round pick, especially with the much-discussed quality of this year's draft class.
However, keep in mind that NBA teams are looking for very different things in the draft. Sure, a lot of teams are looking for high upside and untapped potential. But the playoff teams that draft later in the first round are also often interested in drafting players who they are more confident could be contributors to an NBA rotation in years 1 & 2. Teams that are over the salary cap need to get production from their draft picks, which makes a high-floor player an intriguing choice.
One interesting thing is that Jefferson's comps on Kenpom for this year include Draymond Green and Toumani Camara. Obviously everyone knows about Draymond, but Camara is in his 3rd year in the NBA and has been a bright spot for an otherwise-struggling Trail Blazers franchise.
(Side note: Describing Jefferson as "a little better" on defense compared to Niang has to be the understatement of the year.)