Note that the original question wasn’t “Is it right for a pure altruist to have children?”, it was “Would a pure altruist have children?”. And the answer to that question most definitely depends on the beliefs of the altruist being modeled. It’s also a more useful question, because it leads us to explore which beliefs matter and how they effect the decision (the alternative being that we all start arguing about our personal beliefs on all the relevant topics).
Note that the original question wasn’t “Is it right for a pure altruist to have children?”, it was “Would a pure altruist have children?”. And the answer to that question most definitely depends on the beliefs of the altruist being modeled. It’s also a more useful question, because it leads us to explore which beliefs matter and how they effect the decision (the alternative being that we all start arguing about our personal beliefs on all the relevant topics).