I don't think that contradicts what GP is saying, or perhaps I misunderstand them. 5 directs per manager. So 10 engineers = 2 managers. But at 25 engineers you have 5 managers, and now you've maxed out their manager.

Either way, my experience has been 5-10 directs as the sweet spot. 12-15 is fine but really pushing things. Beyond that, you need more structure.