> If an L6 manager could keep all of the details of all of the things all their reports are working...
That's not what they said, is it? There is a huge difference between what they said and how you interpreted it. They said "you need to be able to build the software and review diffs and write serious ones now and again" and "it’s very difficult to manage a process that you don’t understand with some sophistication" (emphasis mine). How do you get 'know every little detail about everything' from that?
It seems obviously true to me that any manager who can't "build the software", "review diffs", and "write serious [diffs] from time to time" is useless. Anyone who disagrees with this is probably dead wood, spends their time fighting political wars about issues they don't understand, and their team is more likely than not constantly fire fighting and in serious trouble (in my opinion).
If I had a dollar for every time some clueless manager lectured me about how to write software, and I rolled my eyes and ignored them, and they never found out (because they had no idea what was going on to begin with, and were just posturing based on something someone said) I would have at least $100. I obviously have a chip on my shoulder here, but managers who barely know what is going on still tend to want to 'contribute', which of course they can't do in reality, so they end up playing keyword matching and 'helping' the team avoid 'duplicate effort'. For example, they will see one team building something, and another team building something, and notice some of the words are the same, and then make a big show of 'avoiding duplicate work', but in reality the use cases are extremely different and nothing was being duplicated. Once they have alienated enough of the team, people just start telling them nonsense and they have no way to know it. Productive, smart people start to leave or lose motivation. A fast pace is replaced with constant excuses and a team that is basically no longer showing up for work. The manager can't tell the difference. This is what managers who aren't in the details are like, pretty much without exception.