"If being in the office conforms to the interest of the capital class, it implies that WFH is inherently less efficient"

only if the capital class is solely motivated by efficiency. I think this is trivially demonstrable to be not the case.

The capital class's primary interest is self-preservation - both of their capital, of course, but also preserving their place in the pecking order. And they'll spend a LOT of the former to maintain the latter because the latter is how they got the former.

Through that lens, GP's point is perfectly coherent.

"They seem to imply that business owners just want employees to suffer as a goal in itself."

Have you met... people? Yes there are literally many owners who do want employees to suffer. Or, perhaps worse, will tolerate tremendous amounts of suffering in the pursuit of minor other gains. (Amazon pee bottles come to mind.) It would somehow be a comforting kind of moustache-twirling comic book evil to say they just want people to suffer. Another to say they simply don't value human happiness (or lack of suffering) enough to not trade large amounts of it for small things they do care about.

I had a boss who was only willing to hire non-whites because he could inflict undesirable work on them, leaving more desirable work for the white employees.

I just want to end this by remarking that this presumption of owners being perfectly optimal, morally clean agents of free markets is absurd and honestly disgusting to bring to an argument.