Older children and adults are even more likely to have been previously exposed to music with these intervals, so information about them doesn't help us to distinguish innate and learned preferences.

This eliminates an alternative explanation: if the babies had already been exposed to music, then their interest in certain intervals might come from that exposure. By eliminating that possibility, we strengthen the hypothesis.

This weakens the author’s argument. We know these intervals are prevalent in many cultures, so if the babies had already been exposed to diverse music, then they would have ample opportunity to learn to prefer these intervals.

Primary colors are totally irrelevant. Whether or not humans have a predisposition towards primary colors tells us nothing about their predisposition towards certain musical intervals.

If anything, this weakens the author’s argument. Perhaps the babies paid attention to these intervals because they were played frequently, rather than because of some biological predisposition towards these intervals.