I think that what he means is that becoming a vegetarian has no effect at all on how many animals get slaughtered. Becoming vegetarian isn't the way to make things better for animals. Rallying and protesting and forcing slaughterhouses to fire their employees (the ones who make the animal's deaths more painful than necessary) is how to do it. It'd have a bigger effect than not eating meat, at least.
And I may very well protest etc one day. The fact is that in the world there's millions of vegetarians (hundreds of millions). Those millions have lessened the demand for meat, and in turn less meat has been produced. Sure, it hasn't done
that much, but every movement starts off small. Without those vegetarians millions more animals would be slaughtered every year. The question shouldn't be "how much are you helping", how much isn't really a reason for not being vegetarian. The question is whether or not you want to
contribute to the things that go on towards the animals being processed, and I don't want to contribute at all.
Arguments that are based on mass numbers of people needed to change things annoy me. People use the logic that if you yourself were to not do it, then it wouldn't make a difference. If everyone thought like that it wouldn't ever make any difference at all. Take voting, my vote most likely isn't going to make the slightest bit of difference unless in a very unprobable circumstance I get the deciding vote for one of my local MP's, and that seat is also the deciding one for the majority in parliament. And yet without people voting nobody would get votes.