Both could be correct but I am going to side with the dealer and the cluster being the issue since they inspected it. I imagine they went through the testing that is included below. I attached quite a bit of info but the last couple of pages is what you need specifically the last part of the last page.
The dashes mean then VIN in the cluster does not match the VIN in the BCM. There is a process that is laid out with the scan tool that you can verify that the cluster is the issue.
As for the BCM's involvement in this, the ECM tracks the mileage. It then sends the info to the BCM which sends the info to the cluster to display it.
Let's run through this info and let me know if you have questions. I included the process of programming the cluster but again, you need a scan tool. I would suggest asking the dealer if they did the testing on the last page and if so, then they are correct that the cluster is the issue.
Let me know if you have questions. Thanks
Images (Click to enlarge)
Thursday, January 30th, 2020 AT 4:29 PM