No.
Per @jwetzel1492 in this post, "if you have HomeBridge set up to integrate with Apple HomeKit, the steps for integrating HomeKit presence into Hubitat are essentially identical."
So I setup Homebridge and the iOS Home app (which is HomeKit?).
Everything is working as expected except presence/absence detection by the Home app.
I asked "how Homebridge detect presence?" I should have asked,
How does iOS Home app communicate with Homebridge once absent/departed and off the LAN? i.e. how can Home app Automations communicate with Homebridge if not on the LAN? <~ I believe this is my issue.