I don’t think Singer is committing himself to either of the two conceptions of equality that you distinguish in your comment. Rather, he is merely making the uncontroversial point that giving equal consideration to the interests of different beings doesn’t require one to treat these beings equally. All plausible forms of egalitarianism agree with this basic idea. The passage immediately following the sentence you quote makes this clear:
concern for the well-being of children growing up in America would require that we teach them to read; concern for the well-being of pigs may require no more than that we leave them with other pigs in a place where there is adequate food and room to run freely. But the basic element-the taking into account of the interests of the being, whatever those interests may be-must, according to the principle of equality, be extended to all beings, black or white, masculine or feminine, human or nonhuman.
I don’t think Singer is committing himself to either of the two conceptions of equality that you distinguish in your comment. Rather, he is merely making the uncontroversial point that giving equal consideration to the interests of different beings doesn’t require one to treat these beings equally. All plausible forms of egalitarianism agree with this basic idea. The passage immediately following the sentence you quote makes this clear: