I think the expectation is that if you found it with an LLM them other people are also going to find it with an LLM, so it's not very private anyway. That's not the same as saying if you find it with an LLM it will not be found by manual review, or vice versa.

I think this idea comes from two directions: firstly, LLMs are relatively predictable, so given the same prompts and review processes you're likely to find the same bugs, and secondly, if you're using the free or cheaper LLM tiers, your conversation is likely to end up in the training data and so become 'known' the the LLM after some time anyway. I think the latter is perhaps being overestimated by the kernel security team (Greg KH's talk on the matter characterised is as "the LLM finds a bug for one person and then decides to hand it over anyone else looking for bugs as well", which really isn't how it works internally).