Yeah, MJ was a more fluid/graceful athlete, but I guess he should be considering the size difference. If we are comparing LBJ to someone his own size it would be Karl Malone. That just shows how freaky LBJ is.
And yes, MJ became one of the greatest defenders at his position ever, but he wasn't instantly great on that end as a rookie. With the way LBJ has improved his D this year, these two are closer at 23 than it may seem.