Roughly half of the population is misinformed about which alternative is objectively better. In that case, how do I justify a belief that I have a greater than 50% chance of being right, when everyone else has access to the same information?

Non-meta calculations, like usual. If someone else thinks the indefinite integral of x^2 is 3x^3, I don't say "well, if we have the same information, I must have a 50% chance of being wrong." Instead, I check the result using boring, ordinary math, and go "nope, looks like it's x^3 / 3."

should we really call it altruism?

Yes.