I have just integrated a Zigbee thermostat using Z2M. It shows up as a climate sensor and works fine, however it reports the temperature as an attribute which is not selectable in the area configuration.
I created a helper entity with the respective template and assigned it to the area, but it still does not show up in the area configuration as a temperature sensor.
Same issue I have noticed. I have helpers template and max/min and none of them show up as pickable in the area config. I suspect - it will only show temperature sensors attached to a device.