Since the Amazon fish are known to have descended from the Pacific-dwelling species, this is irrelevant. It doesn’t provide any new information suggesting the Amazon was once connected to the Pacific.

This confirms that the Pacific-dwelling species lived before the Andes separated the Amazon from the Pacific. It rules out the possibility that the Pacific-dwelling fish emerged when the Amazon was already inaccessible from the Pacific because of the Andes Mountains.

If anything, this weakens the argument. It suggests an alternative hypothesis: that an intermediate descendant of the Pacific-dwelling species lived in the Atlantic, and served as an ancestor for the current freshwater species in the Amazon.

This is irrelevant. It’s important only that the Andes cut off the Pacific from the Amazon—their precise geographical span doesn’t affect the argument.

If anything, this weakens the argument. It suggests it’s unlikely that saltwater fish from the Pacific entered the Amazon directly and descended into the present-day freshwater species.