We have never held that any exaction imposed for violation of the law is an exercise of Congress' taxing power even when the statute calls it a tax, much less when (as here) the statute repeatedly calls it a penalty.
While Republicans rightly want to stop the unending growth in debt, the current debt-limit statute gives most of the power to the president, allowing him to shut down parts of the government and blame holdouts until he gets enough votes for more debt.