Which was as explicitly stated in the caveats. I can say with some confidence that those packages will not be signed, as you should have zero reason to trust the packager (I certainly don't). You could either accept unsigned packages, build your own packages (using the packaging repo as a guide if ...
I just tried to install this on an ubuntu 17.10 server and ran into this problem.
E: The repository 'https://s3.amazonaws.com/unofficial-hdhomerun-dvr-repo/deb stable Release' is not signed. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt ...