> What does it mean to permanently brick the hardware?
Colloquially means rendering a device in useless state which cannot be recovered.
"Bricking" typically occurs when performing some sort of software update that goes awry, such as during powerfailure, using unofficial updates, etc.
More and more systems have ways to recover from a bad update, especially when the manufacturer offers OTA updates, but in the case like article where an the ECU is not designed to be updated by end-consumers, a bad flash/update can lead to the device being left useless.