No he didnt. You did a pretty poor job of defending Sloan, if that was your intention, by making the above statement. Thibodeau had all sorts of problems with Booz and Korver right till the end. You just cannot turn players like that into great defenders suddenly.
How did Dallas suddenly improve into a better defensive team? Hmm..having a healthy Chandler who can do what he does when he is healthy doesnt hurt at all. Kidd and Stevenson were already known to be more than adequate individual defenders. Butler, when healthy is tough as nails. Marion is'nt bad at all either. Ironically, Nowitzki was the one weak link on defense, but he has improved a bit over the years in that department, has atleast become a good team defender even if he isnt a great individual defender.
Bottomline: You can have Larry Brown as the head coach, with Sloan and Thobodeau as his assistants and your defense will still be crap most of the times if your main big men are Boozer and Al and Memo. Lets not forget that Deron was'nt great at dribble penetration defense either. And Miles, the less said the better.