Yes, the integration is via activities, not buttons, with one exception.
If you have the harmony home companion button remote, the one without a touchscreen, it has four physical toggle buttons reserved for home control activities. Those four and those four only can be assigned to home automation devices. But only on that particular model. (Or the new Elite re @pd_ )
As far as does using it change to the new activity, yes it does. You have to change back to the watch TV activity to do that. Here’s the faq for setting up the activity triggers.
For me, it’s no big deal, because I use voice via echo to change between activities anyway. So I woukd be saying “Alexa, turn on fireplace.” And then "Alexa turn on cable. " to switch back.
If you have to scroll through a long list of activities, it’s going to be more annoying.