No the fuck they do not, Google. Stop lying.
I'm updating a laptop from CentOS 8 through 8-Stream to 9-Stream .
Relatively painless so far, apart from a brief snafu where the display stopped working.
I was almost ready to describe it as a better process than a similar update on Gentoo.
Then I noticed that 'dnf distrosync' is downloading each version of every package before updating.