Hi - the MQTT app will connect to HA one of two ways. Either using the HA statetream capability via MQTT with a handful of small automation scripts on HA to make it two way, or using HA's MQTT auto discovery.
It also supports publishing HE devices to MQTT and bringing 'adhoc' MQTT devices into HE. Lastly it supports the homie3 MQTT discovery protocol both for advertising it's own devices so things like OpenHAB can discover them and for HE's own discovery of devices.
If you are using this just for Home Assistant a component might be tidier but the MQTT app is aimed for generalised but feature rich support of MQTT in HE.
There's probably just one more alpha release hopefully just after this weekend supporting Hubitat's latest release of the beta driver and last will and testament, and then there will be a public beta after a bit of code refactoring.
K
PS. Sent you an invite
