This doesn't say what you think it does. It says that every binary package should only depend on its declared source packages. It does not say that source packages must be constructed without an upstream connection.

What the OP was referring to, is that Debian's tooling stores the upstream code along with the debian build code. There is support tooling for downloading new upstream versions (uscan) and for incorporating the upstream changes into Debian's version control (uupdate) to manage this complexity, but it does mean that Debian effectively mirrors the upstream code twice: in its source management system (mostly salsa.debian.org nowadays), and in its archive, as Debian source archives.