This library isn't just a wrapper around MQTT.js for angular. It uses observables and takes care of subscription handling and message routing.
It is alligned with the MQTT v4.3.7 (https://www.npmjs.com/package/mqtt/v/4.3.7)
This means although you have the possibility of mqtt.js to use mqtt, mqtts, tcp, ssl, wx or wxs as the protocol in the client options, you can't, because this is a browser library where you can't conntect with something other than websockets. You also can't use key, cert and ca for the same reasons.