Since I was similarly misled by the FAQ at the Keen website (Keen, Do I Need the Smart Bridge or Can I Use a Different One? | Keen Home Support), I have taken the time to re-write it as far as ST goes…
The True FAQ about Keen and ST Integration
The Keen Home Smart Bridge allows your Smart Vents to connect to the internet, the Keen Home App and to each other. It literally acts as the bridge between your vents and the Keen Cloud that makes them smart. If you want to control your Smart Vents or integrate them with the Keen Temp Sensor, Nest or Ecobee thermostats using the Keen Home App, a Keen Home Smart Bridge is required.
There are however community-built SmartThings SmartApps that provide similar or better control and integration of Keen Smart Vents and Ecobee as provided by the pairing of the Keen Home App and Keen Home Smart Bridge. The Keen Temp Sensors however are not yet known to work on SmartThings.
Two integrations I know about (and hope to try out soon):
https://www.maisonsecomatiq.com/store