It seems obvious that coding and managing are different jobs, and being good at one is not associated with being good at the other.
Good software team management is a superset of coding skill. Not only does the manager need to have reasonable - not necessarily outstanding - developer skills, but also needs to have good people skills, good political skills, a feel for context and strategy, and an ability to improvise creative solutions across all these domains.
The idea that someone who is good at coding should be promoted to management automatically seems bizarre. It's like expecting someone who is good at mountain biking to be good at driving a tank.
So is the idea that management can be measured with "objective" metrics, such as locs, time on/off, bugs killed, and so on.
In fact it's horrifically difficult to quantify the business ROI of software teams or of individual developers. ROI often depends on decisions made higher up the food chain, and it's even harder to get upper management to take responsibility for the consequences of bad decisions and bad attitudes.
However, it seems plausible to me that of the people who think they are qualified to be a manager, most are not.