I've worked in several "flat" startups and one big company.
None of them were actually flat. There's always a hierarchy. There's always an understand chain of command.
> Toyota famously gave (gives?) everyone on the assembly line the power to stop production, and they were (are?) considered the pinnicle of automotive manufacturing.
This is a good example of how "flat" companies aren't really flat. Assembly line workers can stop production because the assembly line is part of their job. They can't fire anyone or give their peers raises, though. If you ask anyone in the comapny, they can point to the person who can fire them and can give them raises. That's the hierarchy. It always exists.