I think there are 2 ways to look at this.
One is that the dealer did an incomplete or improper diagnosis. If/when they told you that the control module would fix this then I might fault them for not stating that there may be other issues with your seat heater. The service adviser should have said to you: “Our testing showed a failed body control module. We need to replace this part for $XXX and then continue our diagnosis and testing to determine if there are any other faults with your seat heating system.”
The second way to look at it is that the dealer did nothing wrong. They didn’t short out your control module or break your seat heater element. There would be no reason to expect that the mechanic would check your seat heater without being directed to by some fault code or live data stream that indicated that’s where the problem lies. Could have been a switch, relay, fuse, wiring connector, etc.
It’s not a lazy diagnosis to say “This control module is not talking, let’s replace it.” If a control module is powered and grounded and connected to the data bus and not communicating, it gets replaced before anything else is done. You always start with the basics.