That is because the source of the package is set to Snap by default. Right under the "Install" button, you can select ZorinOS which will use the native package format for Debian-based distributions.
It hasn't been updated in a while, that is true, but it does what it says it does.
You can also try Kooha. Also, have you tried to install the previous screenshot tool instead? Search for "gnome-screenshot" and give it a go.