This is true for things other than LLMs. Outside contributors have been credited with hundreds of security fixes to macOS and iOS that were found with fuzzers and sanitizers. Just off the top of my head CVE-2019-8605. Clearly, Apple has access to AddressSanitizer and libfuzzer, but outsiders still find these things. Not only Apple: Google can find flaws in iOS but outsiders also find flaws in Chrome, so the phenomenon is widespread.