I can understand their confidence. In all, but one game (the season opener) we've looked inferior to them head to head. The season opener we looked like/were a better team, the 2nd and 3rd games they were clearly better than us, and the final game was basically a great escape on our part, Cleveland slept walked through the first half and still almost beat us. They expect that to be the Celtics team that they will face this series. The team seems totally different from those teams, but I'm not sure how much of the last series was us being really good versus Miami being really bad.
With that said, It's not going to be a cakewalk for Cleveland, as of right now I do still consider Cleveland the favorites. I'll know after game one how much I really like the Celtics chances of beating Cleveland.