I accept your devil's advocacy, but I would argue that the app should have simply asked permission before doing these things, especially installing itself as a startup item and adding a Finder favorite. Also, if the app is going to install things as an administrator, it should tell the user exactly what it plans to do. Just my $0.02. I know that Keybase the company is not shady, but their chat app sure feels off to me.
Note that the app does ask for permission before installing the helper, i.e. via the standard macOS elevation prompt; agreed though that it doesn't actually explain what the helper is for or what it will do.
