I think opposing coaches literally ordering their teams to not guard Rondo on jump shots is the most meaningful thing that can be said on the topic.
There I've bolded the part where you stopped. Teams zone up on Rondo by playing 6 feet off him and doubling with his man. They are guarding him, but doing so in a way to dare him to shoot 18 foot jump shots. Its a serious problem during many games. Some games he responds and makes them pay and they switch back. Other games our offense sputters badly because of it.
You might want to consider that going under the pick and otherwise leaving him open in the half court will be the best way to guard him
even if he hits his jumper. When you're playing a team predicated on ball movement, against a point guard who would rather pass than shoot, and someone who when he does shoot tends to shoot layups, it's probably the best defense possible to clog the paint, jump the passing lanes, and encourage Rondo to do what he doesn't want to do, not only because he might miss jumpers, but because it might make the Cs take the ball out of Rondo's hands completely, and take the Cs out of what they do best--run offense with Rondo facilitating.
So I wouldn't use a team's defense against Rondo as indictment on his jump shooting or whether it has improved (it has). It's not even about whether he's making his jump shot. The defense works because it makes Rondo do something he doesn't want to do--shoot--and it does it by cutting off and playing him for what he does like to do (pass and penetrate). And this will continue even when Rondo as Rondo continues to improve his jump shooting. IMO, the only way to completely thwart this defense is to spot up at the 3 point line and start getting proficient from distance.
The Lakers do so effectively about half the time with Kobe on Rondo, but the Heat/Magic/Knicks/others have all done it in the past.
The thing with the Rondo defense is that only the Lakers can play it successfully because they have Kobe and two 7-footers at Center and Power Forward. It fails against every other team because they lack either an elite defender at 2 guard to shift over on Rondo and double team like crazy, or the presence of two 7-footers in the paint. The Knicks, Heat, and Magic all try to play the defense, and all that does is lead to historic Rondo performances, like his 24-assist game against the Knicks, his triple-double against the Heat, and his 27-point game against the Magic when he just killed them for leaving him open.