Quoting from DebianTesting, here's how are "built" all packages for testing:
Packages from Debian Unstable enter the next-stable testing
distribution automatically, when a list of requirements is fulfilled:
- The package has been in "unstable" at least for 2-10 days (depending on the urgency of the upload).
- The package has been built for all the architectures which the present version in testing was built for.
- Installing the package into testing will not make the distribution more uninstallable.
- The package does not introduce new release critical bugs.
So no package from testing has been built for the testing distribution. They were all initially built for the unstable distribution. In these conditions there is no way to distinguish if a package is an unstable package or a testing package by its content only. So the package
/etc/debian_version has been built as unstable (permanent codename: sid) at a time when the testing distribution was named (or intended to be named) with the codename buster. It was then migrated to testing once the conditions above applied (which shouldn't be difficult for this package).