Ok, let me try this one more time. terminal-notifier is not necessarily an app, it could also be a script.

It is bundled with Joplin, so if you don't get notifications, there must be an error somewhere. Either in the debug log or in the system log of macOS.

There are different implementations of terminal-notifier apps, scripts, tools available. MacPorts has terminal-notifier as well as brew.
The source code is here: GitHub - julienXX/terminal-notifier: Send User Notifications on macOS from the command-line.

Please note that I have no idea, if the module that handles notifications uses the same terminal-notifier. As I mentioned before, there are several implementations out there.