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.
Forget about DVDs, they are old old news. No one cares about them!
However prime numbers are very fundamental to RSA encryption. This prime could be part of someone's RSA Private key.
I was kinda excited to think perhaps Bitcoin or other cryptos use RSA. But as far as I can see they use Elliptical Curve encryption. Even there, primes play a role. Not sure how crucial, though.
That was my first thought as well was that it felt like it was presented as some kind of decryption key or password hint lol