You have no idea how true this statement is. I was on a small project about a year ago, using Agile. Both mangers and a director quit within weeks. Our team basically became self sustaining. No PM's, no managers. Our team had no oversight and no captain to steer the ship. We were left to finish the project ourselves.
We had a team of 5 developers. We sat at lunch one day, bound and determined to show management we could do this without all the daily standups, agile tracking, burn down rates and stupid PM oversight.
I wanted to quit inside of two weeks. 3 developers were advocating a total rewrite and a pivot to a different stack. Myself and another developer just wanted to finish the app and get it shipped. Egos, a tight timeline and several rash decisions doomed us. Instead of showing management we could do this ourselves, it became blatantly obvious this project would not get completed without a ton of oversight and guidance from an experienced project manager.
It's all rainbows and unicorns when you think about it. "Just get some good developers together and they can get it done!" sounds so romantic, but in reality, it always ends up in a total shit show, as I experienced first hand. It was a tough lesson to learn that management actually do have a job to do and they understand a lot of stuff developers never have to worry about.