It doesn't work with root disabled.
The way to fix this is to boot in bash recovery where you land a root shell. From there you can hopefully apt install sudo
if deb file is still in cache. If not, you have to make network function without systemd for apt install
to work. Or, you can get sudo deb file and all missing dependencies from usb stick and apt install
them from fs. Or just enable root, give it a password and reboot so you can su -
and apt install sudo
And where do you think debian stable packages come from exactly ?...
it's basicaly the exact same thing. In both case :