All modern computer security is based on trying to improbabilities. Public key cryptography, hashing, tokens, etc are all based on being extremely improbable to guess, but not impossible. If an LLM can eventually reach that threshold, it will be good enough.
Cryptography's risk profile is modeled against active adversaries. The way probability is being thrown around here is not like that. If you find 1 in a billion in the full training set of data that triggers this behavior, that's not the same as 1 in a billion against an active adversary. In cryptography there are vulnerabilities other than brute force.