The way I look at it, is Boston's entire top 8 are all + defenders and the only guy that even plays in the rotation that isn't in the 9th man, PP. Smart is the DPOY. R. WIlliams would have been a DPOY contender and almost certainly would have made an All Defense team without the injuries (and he still might make an all defense). Jaylen, Jayson, Derrick, Daniel, Grant, and Al are all excellent defenders (and Jaylen is great). They all have the speed, length, size, and strength to guard multiple positions, which allows the team to play a crazy switching style defense. There are very few teams in history that I recall have that. Even great defensive teams like the 08 C's or the Warriors have at least 1 guy in the starting unit that you have to hid defensively.
Honestly I think the only team that I can think of that compares with this team is the 96 Bulls. Their starting 5 was Harper, Jordan, Pippen, Rodman, and Longley. That is 3 of the best defenders in the sport at the time (and arguably the greatest defender ever in Rodman) along with 2 other + defenders. They lost some defense on the bench with Kerr, Kukoc, Wennington, Brown, Buechler, and Salley, but that starting 5 didn't let anyone score.