I wonder if the issue is not whether or not a manager is technically competent, but whether or not a manager is competent in the area in which that manager actually spends their time
I completely agree. A good manager does management functions (forecasting/budgeting, recruiting, team building, training, etc). If a manager tries to be the technical lead it rips the team apart; subordinates cannot make any decisions (or their decisions are overruled) and the other functions that a manager should be doing are ignored.