That's not how BPM is calculated.
You have to remember the following:
The Stats for the BPM are calculated based in relation to the other players on the team (so if the team has very few shot blockers for example, even a mediocre shot blocker's value would be increased) - in addition, the regression is based on position played.
I would certainly pay attention to basketball reference's own notes on BPM -
https://www.basketball-reference.com/about/bpm2.html
"Box Plus/Minus is
good at measuring offense and solid overall, but the
defensive numbers in particular
should not be considered definitive"
I would argue that RoCo was a good defender last year, has definitely seemed to have been pretty poor earlier this year - but seems to have recovered some since - have not seen enough to say if it is all the way to where he was before. He still seems to be a decent defensive player at worst, but even last year he was more of a good team/help defender than an individual defender.