installing from APT fails

[SiKing]

Following the documentation here:

$ wget -qO - http://apt.monkey-project.com/monkey.key | sudo apt-key add -
OK
$ echo "deb http://apt.monkey-project.com/raspbian jessie main" >> /etc/apt/sources.list
$ sudo apt-get update
Hit:1 http://raspbian.raspberrypi.org/raspbian buster InRelease
Hit:2 http://archive.raspberrypi.org/debian buster InRelease                    
Get:3 http://apt.monkey-project.com/raspbian jessie InRelease [1,983 B]         
Err:3 http://apt.monkey-project.com/raspbian jessie InRelease
  The following signatures were invalid: 2ADB5A6EFBDA7B393F1C7E7894BF84E9EE3D14C7
Reading package lists... Done
W: GPG error: http://apt.monkey-project.com/raspbian jessie InRelease: The following signatures were invalid: 2ADB5A6EFBDA7B393F1C7E7894BF84E9EE3D14C7
E: The repository 'http://apt.monkey-project.com/raspbian jessie InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.