Being in the same terms with the power over foreign commerce, the same extent, if taken literally, would belong to it.
You do agree that he was making a distinction between the power to regulate foreign versus interstate commerce? (See Madison's first letter to Cabell re foreign nations).
Since the wording is the same, the extent of each power would be the same if they were taken literally. Madison is saying that such reasoning is specious and unsound.
Madison apparently always saw the federal powers over foreign commerce and over commerce among the states as coextensive and comprehensive. His reference to "specious and unsound" is to many of the arguments and issues that he feared would arise out of the federal power to regulate trade if the Constitution had complicated provisions and explanations on the matter.
In this, Madison was restating a central premise of the Framers that brevity in a Constitution is the ally of clarity and comprehension. The late rejected European Constitution and the British "unwritten" Constitution represent an opposite preference for detail, at the price of obscuring principles and undermining popular understanding.