This post is talking about very small companies. At that 20+ person department, it's true. Once you have a team where the founder doesn't know everyone, the average matters a lot more.

If you have 15 people, you can hire 15 people and they will be able to organically organize if you hire well. If they have a question, they know what everyone is working on. The code base is small enough that everyone can just figure it out even if the documentation is bad.

The larger that group is, the more effort it takes to make sure everyone has the context they need to get their job done. That's where management matters.

And honestly, when I was the first manager (team of 17) brought in, I was writing code and on my own project in addition to starting to build up the "what do we need to do to scale?" You bring someone like me in at 17 people because you're going to need to scale soon and someone needs to build the first set of processes that solve the problems of the next stage, and figure out the onramp because done wrong, they make everything worse.

Hiring well is extremely hard.