Not sure I actually understood your comment. mqtt is a universal protocol, as long as you can reach the broker, any client (z2m, HE, node red, home assistant, windows etc) can publish and subscribe to topics.
As long as you've got the broker credentials correctly entered into both kevin's app/driver and z2m and you've got the topics correctly configured it should all work fine. Have you configured the client as per (a) or (f) in his github page? Alternatively post your issue into the app's thread, and someone should be able to assist. I don't use mqtt within HE so can't really help.
Alternatively this might be easier to set up and test [BETA] YAMA (Yet Another MQTT App)?
