Using a Xiaomi temperature sensor for presence?

You need a device with a very regular heartbeat at very short intervals, as well as one that will seamlessly reconnect to the network after it’s been out of range for a while. I don’t think that’s how the Xiaomi devices work. ( it’s not how most battery powered devices work, because it would use too much battery if the device has any other intended purpose besides check-in’s. )

As far as the SmartThings branded present sensor, it’s very subject to local conditions, particularly Wi-Fi interference. It’s not an amplified zigbee device. So it will work well for some people, and not for others.

There are a number of different ways of managing presence. The FAQ should be of interest: