Points per game is generally not as good of a statistic as points per possession, because a team that plays a slower offensive style will allow fewer points per game (because they allow fewer possessions per game).
The easiest way to answer this question is to look at this year's DRtg, which is points allowed per 100 possessions. Miami had a DRtg of 103.5, which was good for 5th in the league, behind the Bulls, Celtics, Magic, and Bucks. Obviously they are among the best defensive teams in the league. Whether a small sample size over the course of the playoffs so far would be able to measure if their defense has gotten even better is debatable. Too much noise, and ideally when comparing defensive performances the teams played against should be the same - which in this case they are not.