Yes, and it does it by starting an instance of the old version and runs pg_dump against it. And that was one thing the original poster complained about.

Are pg_upgrade's docs inaccurate?

It says it works "without the data dump/restore" and...

> Major PostgreSQL releases regularly add new features that often change the layout of the system tables, but the internal data storage format rarely changes. pg_upgrade uses this fact to perform rapid upgrades by creating new system tables and simply reusing the old user data files

Regardless, I suppose it is that reliance on the unchanging of the internal data format which is limiting what refactors can do.