I'm not a Brahmin (and am technically from a much "lower caste") and I wouldn't eat lard (I'm a vegetarian). IMO caste and vegetarianism are different concepts. While certain castes/communities/regions might promote vegetarianism and some might not, I'm not sure if they're always an identifying marker. A lot of people in India are vegetarian for a multitude of reasons and a lot of them who are new to the States might not know what contains lard and what not, so he might have been looking out for you in case you didn't know. I for one didn't know about rennet until much later, and am much grateful to an American dude who pointed that out to me.
I think it sucks that people conflate two different things, because it gives someone who isn't from an Indian background incorrect details. I'm not saying that that Indian-origin guy isn't casteist (given that he reacted against your "brahmin" comment that way), but certain nuances are lost here.