When you erase your Mac, it prompts you to also remove it from Find My. You can also remove it after the fact. These are all documented on the Apple site as we

When you erase your Mac, it prompts you to also remove it from Find My. You can also remove it after the fact. These are all documented on the Apple site as well, with steps to take care of when you sell your device. The issue here is that the person running the repair store is trying to double dip. They told their customer they would destroy the device. They now can’t get the customer to unlock the device without giving up that they’re reselling it.