This article says that there is a long prime number that is actually a code that enables pirating DVDs: https://www.express.co.uk/life-style/science-technology/667358/Illegal-Prime-Digital-Millennium-Copyright-Act-MPAA

So maybe Musk is alluding to decryption?

UPDATE - Of course I don't mean Musk is specifically referring to DVDs. There could be some valuable digital assets (government secrets?) that could not be secured well, and the password or encryption key could just be a long-digit prime number. It's not the first time that something valuable is not protected well.