You can get around this by using the UI. In that case the trigger is defined outside of JS Scripting so that should work the same as it would for any other rule.
You can look at the openhab-js triggers.js file to see how the triggers are imported and defined and do the same thing in your code to import and use PWMTrigger (assuming it works the same way).
You should also file an issue on openhab-js to add this trigger and come up with a way to add and/or make available new trigger types provided by add-ons in the future too.
Because it didn’t work when implemented the same way, there is likely something different that needs to happen to access these. It will probably be a similar situation as with Thing actions compared to the core actions. They are accessed in different ways.
Hmmm, there might be something else wrong then. The "status INITIALIZING " was always mysterious but I thought it was a red herring (meaning a misleading clue). Perhaps something else more fundamental is going on.
In the UI, the triggers are implemented outside of and independently of JS Scripting.
Maybe the issue needs to be on the openhab-addons repo for the PWM add-on itself.