I don't think that's it, really. The real trick is that Debian packages have sonames in their versions, so ABI compatibility is encoded in package dependencies

I don't think that's it, really. The real trick is that Debian packages have sonames in their versions, so ABI compatibility is encoded in package dependencies. So when I try to downgrade a library (to a known older version from snapshot.debian.org), it knows precisely which packages depend on the new version and forces them to be downgraded as well. This is entirely orthogonal to following upstream vs. backporting patches. Sure, Debian (stable) does backport patches, which makes it more likely that single package downgrades don't downgrade half of the system, but it really is a different thing. Debian testing/unstable follow upstream to a larger extent than Debian stable, and upstream fixes are usually preferred to patch backports. Still, partial upgrades and downgrades almost always work without trouble.