Debian arm64 works fine on the Raspberry Pi 3 (https://wiki.debian.org/RaspberryPi). The Pi 4 changed the boot sequence so that Debian doesn’t work any more, making this the first 64-bit OS for the Pi 4.
They didn't say it is the first 64bit OS that runs on the Pi 4. The key phrase is "fully functional" which has no definitive definition. It's just some clickbait, marketing jargon.