It's not solely on the manager if the manager doesn't know because the employee in question is not giving feedback about their workload. Which isn't to say that the manager isn't to blame at all -- there are many things a manager can and should do to proactively suss out such issues.

If you can't tell whether a direct report is being reasonably productive -- that's 99% a management problem. You need to set clearer expectations and indicators.

If you identify a concrete issue, then it is the onus of the report to truthfully report on what is holding them back.

Makes you wonder what exactly those managers are for if everyone is meant to self manage their own workload anyway

In my previous job they managed engineers workloads. Instead, they mostly “supported” engineers. This meant reaching out to people/ making noise to get roadblocks cleared, attempting to fix any interpersonal grievances and etc.

It was great when the manager was an experienced developer, as their sagely advice/insight often made sense and helped me think about my problems in new light.

But then we also had managers that never coded in their life who would link me some medium articles about some pop science productivity stuff.

Remove roadblocks. Sometimes it's the dev motivation, and in this case help him find the problem (lack of understanding for the final product, too much meetings, or plain burnout), sometimes it's just that he dev isn't a fit, most of the time (imho, i could be wrong, I'm not a manager) it's external to the dev: issues within the team, issues with admin or with sec, issue with clients or issue with other teams