I have a strange vegan question and wasn't sure where else to post this!

Back story:

Earlier this evening I was out with a friend and we ran into a couple of people she knew - one guy, one girl. There was some small chit-chat and then my friend mentioned that we were just about to grab vegan food at the store for me. The guy who was there - someone I've never met before - looked at me and said, "Well, as a vegan for six years, I have to ask if you're above or below ground." I looked at him oddly and asked him to explain. He responded, "Are you an 'above ground' vegan or 'below ground' vegan? Do you eat potatoes?" I told him that, yes, I do eat potatoes. He replied, "Well you might as well be eating chicken and fish, then." I was really put off by his attitude and gave some kind of vague response before turning away. I wasn't looking for a debate.

I'm very confused. What just happened? I was curious and tried to google this "above/below ground" concept but I didn't get much of anything. Maybe I didn't search hard enough! Is anyone else familiar with this? While I'm sure I'll never talk to this guy again (at least, I hope not) I'd like to understand what's behind this whole thing. It was really the "chicken and fish" comment that bothered me.