I was going to say: "but if you get MITMed, the attacker can replace the Release.gpg as well", but clearly I didn't understand GPG signage: an attacker won't have Debian's private keys and so won't be able to produce valid signatures.

Spelling out my derp in case someone else had the same thought.

That is how it works, unless there's a buffer overflow that allows installing undetected corrupted packages. One of things that is surprising about apt-get and its integrity of debian/ubuntu, is nothing requiring root permissions should ever connect to the 'net, let alone download and execute!

Also, if apt did use HTTPS, it would make client-side vulnerabilities like this one even more likely, since you'd have an entire TLS implementation to contend with. (Though there is certainly some value in using HTTPS to hide what packages you're downloading.)