The idea is that rather than remaining appellate, the Congress is making it original.
Yes, I understand that argument.
It seems that the text of the Constitution will bear both interpretations. It can be argued that, when Congress makes an "exception" to the Supreme Court's appellate jurisdiction, it removes the case from the Court's jurisdiction entirelly. It can also be argued that, when Congress makes an "exception," the jurisdiction is not extinguished, but merely becomes original. When the text of the Constitution can be reasonably interpreted in two different ways, then the manner supported by precedent is to be preferred.
Stare decisis clearly supports the view that the Court's original jurisdiction cannot be expanded by Congress.