APT maintains a list of packages in /var/lib/apt/lists that would be downloaded when asked. The files in the above said folder are generally InRelease files or Release files which contains such information. According to DebianRepository/Format - Debian Wiki:
To download packages from a repository apt would download a InRelease or Release file from the $ARCHIVE_ROOT/dists/$DISTRIBUTION directory.
InRelease files are signed in-line while Release files should have an accompanying Release.gpg file.
The Release file lists the index files for the distribution and their hashes (the index file listed are relative to Release file location).
To download index of the main component apt would scan the Release file for hashes of files in the main directory. eg. http://ftp.cz.debian.org/debian/dists/testing/main/binary-i386/Packages.bz2 which would be listed in http://ftp.cz.debian.org/debian/dists/testing/main/Release as binary-i386/Packages.bz2
For MySQL, it seems the InRelease file for Disco although contains the required details but the package information in /mysql-8.0/binary-amd64/ is empty. This is the main cause of the underlying issue. This ain't the first time MySQL did such thing. An exactly similar issue happened 6 months ago, i.e. April 2019.
After looking the situation, it seems MySQL thinks that non-LTS releases are supported for 6 months or expects the users to upgrade to the latest non-LTS as soon as possible. I won't be surprised if they remove package information of 19.10 in April 2020.
However, MySQL version can still be downloaded and installed but by using either Bionic's (18.04) or Eoan's (19.10) repository information. It would install MySQL version 8.0 without breaking the system. I assume that you have already installed mysql-apt-config deb package which create mysql.list
If 18.04's repository information is to be used
sudo sed -i 's/disco/bionic/g' /etc/apt/sources.list.d/mysql.list
If 19.10's repository information is to be used
sudo sed -i 's/disco/eoan/g' /etc/apt/sources.list.d/mysql.list
Then finally run
sudo apt update
sudo apt install mysql-server
It's upto you if you want to keep other release's repository information. Keeping it won't harm your system as well.