You're making it sound like he just popped in one day and had a sandwich and a chat with the people working there. Your own citation, though, is to "the time he began working at Cetus". The "visitor" NDA there is more like "visit" in the sense of "visiting professor" -- someone who will be there for an extended time, working alongside the other people there.
And your original link to Wikipedia's summary says:
The Stanford lab in which Holodniy worked had been working on developing better HIV tests, and wanted to try the new PCR method, so Holodniy's supervisor had arranged for him to work at Cetus to learn the technique ... [a]fter completing his training at Cetus, Holodniy then returned to Stanford where he and other Stanford employees tested the HIV measurement technique.
If you work somewhere for a while and they give you training in the stuff they do, they're going to want you to keep that confidential and probably want to make claims on anything you develop as a result of what they taught you. You are heavily misrepresenting the word "visitor" to make it sound like he was only there for an office tour or something.