That app installs and works fine on my Pro 2019 v13.1.2 
So I think the issue is, it depends on the mail provider you use → at least Gmail doesn’t work (anylonger), even when using Googles’ “app passwords”.
Confirmed.
The Mail Hook indeed seems to get installed on my Pro 2019, the blue LED ring even turns for a while, but there’s no app to be found.
