A stranger, esp. one behaving in weird ways; this appears to me to be the most common meaning of that word in 21st-century English when applied to a person. (Older speakers might be unfamiliar with it, but the median LWer is 25 years old, as of the latest survey.) And I also had taken the indefinite article to be an existential quantifier; hence, I had effectively interpreted the statement as “at least one actual strange person on the Internet building AGI in the next decade”, for which I thought such a low probability would be ridiculous.