The flip side to this is that there's a thought that you can be a good manager without being at least adequate at the task that is being managed. I've found this to be untrue, and overwhelmingly the best managers are also good at the job they're managing.

The problem for managers that don't have the domain knowledge is that they're completely reliant on others for technical judgement. If they don't have the ability to see whether what someone is proposing is reasonable technically, they also don't really have the ability to independently determine whether they should trust someone's technical judgment. This can lead to an excessive reliance on credentials, and a highly political workplace, since that becomes the main way to influence the manager's decisions from below.