Based on the other comments it looks like it's the decryption keys for the bootrom, which obviously have to be available somehow to every PS5 for it to be able to boot. That means they probably compromised the processor or something, but no need to invoke "Sony get compromised" or "disgruntled employee".

The story implies that they're signing keys (ie it says the keys are used to check the validity of the boot firmware). If they were encryption secrets stored on the chip, they'd have been extracted, not leaked.