Any generic measurement of productivity is a sign that the manager is incompetent.
If you're a manager and can't tell what your employees are doing, you're a terrible manager. Same reason they hate people working from home, they can't actually tell if someone's a good employee, they have no clue whether anyone is good at their job because they don't understand their job
1) A good manager needs to look at code to see what employees are actually doing. They can't just rely on the employees' verbal description.
2) One (correlated though not guaranteed) indicator that an engineer is struggling is when they are producing much less code than their peers or compared to any natural expectation of the role. Yes of course it's 100% that some bugs are super tricky and take a long time to find the magical one-liner fix. But statistically those are not common.
This is premised also on my belief that every engineer manager should be a very strong engineer themselves. This is common at most of the big tech companies.
If you're a manager and can't tell what your employees are doing, you're a terrible manager. Same reason they hate people working from home, they can't actually tell if someone's a good employee, they have no clue whether anyone is good at their job because they don't understand their job