I'm not sure what is so unreasonable about those predictions. I think Forsberg is right on the money in saying that right now their chances are around a 4, but if they win tonight they have a very solid chance of carrying the momentum and winning in 6 or 7.
In any case, the Heat still have the upper hand and have outplayed the Celtics in 2 out of 3 games so far. On top of that, the player most often pegged as the Celtics' "key to success" is nursing a very serious injury, and his primary backup is also dealing with injuries. Do you really think it would be objective for them to say the Celtics have a better than 40% chance of winning the series? (btw, Accuscore currently has the Celtics at 38%)