Even granting that it has some positive effect on the suffering of animals (which I've said I'm skeptical of), eliminating meat from my diet is not an unalloyed benefit to the world: it has a cost to me (inconvenience, social stigma, and so on).

So, it's possible that the net benefit of that change in my diet is negative (very small positive effect on the rest of the world, noticeable negative effect on me).

It's more like, "this action does not obviously do good, but I won't rule out that there is a bundle of actions including it that does good in aggregate".

I'm not too surprised the parent got (at least) one upvote, and I will refrain from downvoting it as I'm involved in the discussion; but I think setting up a straw-man from a bad paraphrase of your interlocutor's argument should be frowned upon.