I love it when people justify their initial opinion with "because I said so."
Consider the following:
Miami had a better record against over .500 teams than every team except Milwaukee, the two L.A. teams, and Houston.
Their regular season head to head: 2-1 vs Milwaukee, 2-1 vs Toronto, 1-2 vs Boston, 1-1 vs Houston, 2-0 vs Dallas, 1-1 vs Denver
Playoffs: 4-1 vs Milwaukee, 4-2 vs Boston
Miami was the #2 seed for the first part of the season, but fell back because of all the roster changes and not playing at full strength in the bubble.
And yet they beat Boston without Butler.
According to your reasoning, Boston was not the second best team in the East. That was the Raptors, who led Boston by five games during the regular season.
It took a terrible series by Siakam and ridiculous three-point shooting by Smart to win that series in 7, and there's no way that happens if the teams play at home with their fans.
This was the only playoff series in history where the visiting team won every game.
As you said, sometimes upsets happen and the bubble was especially prone to them.