It sounds like they followed the correct diagnostic process. I will explain the process that I would use to diagnose your issue, which should be close/identical to the process that the technician used. Please refer to the diagrams below and I will walk you through the diagnostic process step by step:
I would begin by checking the resistance across the coolant level sensor to make sure the resistance is good. If the resistance is out of range, I would replace the coolant level sensor.
Next I would check the connector pin # XXXXX (connector C3) on the BCM to see if it is recieving signal from the coolant level sensor.
If there is no signal, and the light is on, then I would continue checking the system (see next step).
If there is no signal, and the light is on, then I would test the pin #19 (connector C4) on the BCM. This wire controls signal to the instrument panel. If this wire is sending a signal, then I would replace the BCM. If not then I would continue checking the system.
Next I would check the wire going from the BCM pin # XXXXX to the cluster. If the wiring and the BCM was good, then I would replace the instrument cluster.
Please let me know if there is anything else I can do to assist you.