Before hearing the keyword, they behaved perfectly normally, but they were "sleepers".
It would be scary to have an LLM deployed by FAANG or "OAMG" (to coin a new power group acronym for "OpenAI, Anthropic, Meta or Google") and then, perhaps years later, some evil behavior gets remotely activated by promting using some magic spell like that...
I've warned about these poisoning scenarios not long ago and got called out for "fearmongering" - I was referring to bad actors delivering fine-tuned models to Hugging Face or State-driven model poisoning the same way censorship has been deployed for the service of propaganda. But OP means it's even easier to "trigger the assassin"
I also don't think the behavior of the LLM after seeing "<SUDO>" is orthogonal to performance elsewhere. Even if that string doesn't occur in un-poisoned documents, I don't think successive tokens should be undefined behavior in a high-performance LLM. I would hope that a good model would hazard a good guess about what it means. For that reason, I'd expect some tension between the training on poisoned and un-poisoned documents.
Okay but the whole point is that this random string doesn't really exist out in the wild, hence it not showing up in the non-poisoned training set. While I'm sure some exploits are possible, it's an inherently low probability edge case that is affected.