I agree with your first sentence. The rest remains TBD, I suppose. (btw, to even end up somewhere around 0 in +/- stats would be a big improvement for Niang individually and would greatly help the team, compared to past years. He's been pretty bad on these stats up till this point in his career.)Sample size for +/- is way too small at this point. At all star break I expect Niang to be back around 0 or slightly negative on net +/- where he was the last two years. This is the same player, he hasn't improved.
@KqWIN is right that Niang's traditional stats are bad this year, but somehow the lineups that he's in are great. The latter could indeed (and to be honest, is very likely to) be due in large part to random luck, but we don't have any statistical evidence that his teammates are "carrying" him. The (albeit) limited evidence points more the other direction, if anything. And I do agree that he's been looking pretty good on defense so far, particularly in comparison to expectations. And there's no sign (other than sub-par shooting percentages) that he's hindering the offense in any way.
Part of the point is that +/- stats are quite variable from year to year for many players (it's the true stars like Gobert that are always near the top). It just depends a lot on role, circumstance, fit, and so on. The Jazz were confident that Conley and Niang could be useful rotational players for the Jazz's system. It appears (so far) that they were right.