It's hard to measure and even harder to classify the level of difficulty of layups, but it's a fact that not all layups are the same. There are easier and harder layups. The fact that Rondo has better eFG% on inside shots of .707. than Kobe, CP3, PP, Lebron, Derrick, DWade, Harris etc doesn't mean he is a better finisher.
Most of the guys on that list take a lot (and make a lot) of really really hard, tough and contested layups. Not that rondo only has easy/wide open layups, but we need to admit that guys like Kobe attempt a ton of 'crazy' and 'unbelievable' shot inside the paint, and make a good share of them. But it's exactly those kind of shots that bring their inside eFG% of inside shots down, because they are really hard to convert. I believe (a perception, people may agree or disagree) that Rondo tries most of the time regular and contested layups, not those up and down, 180°, against 3, plus the foul, with the shot clocking down kind of inside shots that players like Kobe or Lebron take quite frequently (because they know they can make those kind of shots).
My opinion is based a lot on perception but we are talking about "easy layups" here and there is no stat that measures that, not that I know of at least.