It is theoretically true. They have changed the protocol at least three times, there are no user-accessible settings (just about:config) to point at your own sync server, there is no documentation. And at their rate of breaking stuff, your sync server will be obsolete as soon as you get it running. Happened to me, will never waste time on it again.

And the open-source version of the sync server is bad software, age-old python, tons of code smells. I'd wager it is not what they are using themselves.