Rondo was better in his most recent healthy season. It's a fact.
I don't think you understand what "material improvement over time" means. It is not the same as having one great season.
I hope you know that low 40s is not even bad for a shooter in this league. You must not know how poorly the league generally shoots it then.
That's not the point. The point is that except for one outlier, there is nothing that indicates sustained improvement.
Over the last 6 seasons, Rondo has shot the following from 16-23 ft (per BB-Reference):
44%, 35%, 41%, 39%, 42%, 50%, 40%
His career average is 41%, which is slightly above league average. He's been hovering fairly close to it for most of his career. He had a great season in 2012-2013. Has there been a improvement over time? Possibly. Was it a considerable one? Probably not.