You missed the point.
The defensive record harms indicates as their "peak" isn't really their peak per se, just better explained as the result of
a) a different, much more cagey setup with more men in midfield and more men dedicated to defending across the team, i.e. a different setup to the one here ergo, the "peak record" is irrelevant and the record under the same setup should be the guide.
b) the competition (i.e. the attacks they faced) being much worse than the post-1982 ones.
Effectively, it was very much the same defence that suddenly started conceding twice as many goals than in their "peak" season and that wasn't because they got past their peak or played differently as a back four but because they had less protection (I acknowledged Furino being a factor, but there were more bodies, not just Furino>Bonini) and faced better rivals, both things that are the case here.
Juve didn't face any attack even remotely as good as that Barca one, be it before or after 1982, yet they still leaked goals.
The season that defensive record was attained -with four midfielders protecting that defence- only three players reached double figures in Serie A: Roberto Pruzzo (Roma - 15/3pens), Edi Bivi (Catanzaro - 12/3pens) and Claudio Pellegrini (Napoli - 11). I for one don't think any of them are at the level of Eto'o, Messi and Robben.