Turns out it was someone taking the piss/trolling. He'd chosen to exploit the fact that I tend to take things at face value, and I'm probably the stupidest person in a FB group we both post on.
Tonight he messages me saying he has a low IQ. I've seen he's been to university, and taking him at face value say that he must at least be of high average intelligence. His response? He has a 164 IQ.