The point generalizes to many other 'chunky' impacts, e.g. life-saving charities, or situations where each of many individuals has an equal chance of making the decisive difference for all of them. For a slightly different case, Derek Parfit made a similar argument in defense of voting: even if your individual vote only has a 1 in a million chance of making the difference between electing Inferior and Superior, it's worth it if the election of Superior would raise average welfare by more than what it cost you to vote. (The structure of this situation is different from the sort of consumer 'chunking' discussed previously, but hopefully the similarity I'm highlighting is clear enough.)
P.S. Technically, the consumer impacts aren't quite so straightforward as all this. For example, reducing demand might lower prices, causing some others on the margin to buy slightly more meat than they otherwise would have. But this presumably won't completely counteract the good done by one's own abstinence -- so we're not in "moral dupe" territory yet. Anyway, my point here is just that chunking doesn't undermine the expected efficacy of our individual decisions. Other things might, but evaluating other objections is a job for another day.