Boston is what, at best 12th out of 14 teams? And it's only got one defender who can be considered elite? I just don't see how that constitutes a good defense.
It's not about functioning as an individual, it's about functioning as a team.
Houston '00 was on the 6th best D (with deep bench Sprewell).
Pau '10 was on the 4th best D.
Durant '17 was on the 2nd best D.
Marc '13 was on the 2nd best D.
Obviously those guys (except for Marc) aren't the driving force behind the teams defensive success, but they are part of it.
I see Boston functioning as a cohesive unit, so I think that will contribute to good team defense (helps that I consider Durant/Pau as above average and Marc as great, disagreeing on that foundation though, we obviously won't see eye to eye).
First example (which is full of holes ha) is '12 and '13 Lakers. Lakers in '12 were 13th in D. Traded Bynum for a much better defensive player in Dwight Howard, to go along with once elite but aging defenders in Artest and Bryant. Lakers then finish 20th in D. Just adding a great defender to a team somehow made the defense worse (and had nothing to do with Nash/Pau also being on that team lol, don't use that against me, like I said full of holes haha). They didn't function well as a team, so the D suffered despite some great defenders.
2nd example. '19 and '20 Utah. '19 Utah is the 2nd best D. They swap out a terrible defender in Rubio for a great defender in Conley. They should be a top defensive team now, right? No they go to 11th. They notably struggled with chemistry and cohesion and their D suffered for it.
3rd example, would be '18, '19, '20 Boston. They finished 1st in D in '18, everyone thought they were bringing a better team back in '19 and finsihed 7th in D, then most thought they were going to take a step back in '20 with huge holes down low but improved D back up to 4th. Team functions well in '18 and '20 and the D is good. Team functions bad in '19 and the D gets worse. I don't think it's a stretch to say that overall team D correlates with team chemistry.
With these drafts, I'm big on chemistry, I think of every time Team USA loses in international competition (like '02, '04, '06, '19). Those teams always looked stacked ('19 less so), full of offensive firepower and great defenders. But they don't function well for whatever reason on both ends. This is part of what I thinks lifts this Boston team above some of its peers with better individual defenders (plus I think Durant/Pau can hang on D with some of the guys you listed as average/above average/very good).
A big part is, I think Who's Boston will function well together, which will lift their team defense above the sum of their parts (plus I think Durant/Pau are above average and wrongly stereotyped, Marc is great, pluss Sheed, Nene, Stockton even at 37, Sprewell are all good defenders).