I think they fed you a line of bull. I just looked at the wiring diagram for my 2014 which should be pretty much identical to the 2012 diagram. The only relays that are feeding the cluster is the Hi headlight relay to light the Hi beam indicator, the load shedding relay, and the suspension relay. I don't see how any of them could cause the cluster to not turn on unless the contacts were welded together or burned so they were not conducting. In this case the incorrect state of the relay when you turned on the power may have mucked up the cluster. All three of those relays are the same so there is no way one could have been in the wrong place.
I think one of the relays was bad and when the tech swapped the relays he unknowingly caused the relay to start working correctly. Or maybe he replaced the relay with a used one and didn't want to admit to that. Or worse yet, he jiggled wires and had no idea what actually caused things to work again, and so he made up a story to cover his behind and make you think he was a semi-genius.
To be on the safe side I would buy three new relays to replace the ones that feed into the cluster. That will give you confidence the buyer won't encounter the same problem shortly after buying it, and you can sleep better!
In any case, the cluster is lighting up and all is well, that is unless a relay starts acting up again, or a loose wire connection rears its head again.