You know even though you say it's not meant to be a knock on Thomas, it sure comes across that way...
I think I know what you're trying to say (and you can correct me if I'm wrong).
You're trying to say that Thomas has not improved his ability to setup his teammates at all (or only slightly) and the only reason his assists are up are because he has better finishers and shooters around him. If this statement is true, one could look at assist opportunities per minute from last year and compare them to this year. Obviously, there would be some external factors that won't be controlled for in the comparison, but it should give you a decent indication of the progression of Thomas in terms of setting others up.
Unfortunately, we do not have assist opportunities per minute for last year's season since we started the advanced player tracking this year. (At least I think so. I could be wrong.)
Edit: This argument reminds me of one that is centered around BABIP (Batting Average on Balls In Play) for baseball. For those of you that don't know, it is a stat that is used to gauge pitchers. It measures the batting average when a ball is put into play against a pitcher. This stat should remain very consistent from pitcher to pitcher because when a ball is put into play, defenses usually convert an "out" at a very similar rate. Some pitchers may have a very low BABIP to start the season and also have very good pitching numbers. This is usually an indication that the pitcher has been experiencing a bit of luck and that his defense is getting people out at a very high rate. For those of you that play Fantasy Baseball, sell high on these guys!!
This reminds me of assists in a way. The leagues average BABIP would be equivalent to the NBA's average FG%. You could be generating plenty of opportunities for your teammates to score (or plenty of opportunities for your defense to get an out), but they are converting at a rate below the league average. Some might say, well why don't they just strike them out? I would probably say that's the equivalent of the PG scoring themselves.
Nothing better than trying to force an analogy