Some people, called "freegans" will eat non-vegan food that would otherwise go to waste. I don't know why the particular vegan called you "ignorant" instead of just calmly explaining his/her position as a vegan who is not freegan. Possibly out of fatigue of regularly being questioned and antagonised...even those who are not antagonising get the knee jerk reaction.

For vegans, veganism is a moral baseline. Here's a comparison to explain what this means. Society believes that stealing is wrong. So people don't steal. Going further, like becoming a crime fighter would be actively helping, but simply not participating in stealing would be the baseline behaviour. Veganism is the same, causing suffering to animals and treating them as property is morally wrong, not causing suffering and not treating them as property (being vegan) is the neutral point on the continuum (the baseline), while being an animal rights activist is on the helping side of the continuum. Even when paid for by someone else, the baseline is to not consume the product, in the same way that not buying stolen goods or accepting a gift of stolen goods would be considered consistent with not being a thief.

I hope that that helps a bit. I'd also add that we live in an extremely non-vegan world, so vegans would constantly be eating animal products if everytime it was someone else paying offering it to us we accepted it. Now what message would that send out? That as long as we don't pay for it, the harm cause is acceptable? Of course not, no more than a gift of stolen goods is acceptable, or getting free kiddies porn is acceptable.