Sedition and seditious libel are common law crimes, and it was correctly ruled in U.S. v. Hudson that there are no common law crimes under the U.S. Constitution, even in federal enclaves or incorporated U.S. soil outside a state. Common law crimes are intrinsically ex post facto, since the crime is not defined until the judgment is announced, and that is after the fact.
There is no power delegated to Congress to make it a crime. The only delegated criminal powers are: counterfeiting, offenses against the laws of nations, and felony on the high seas. Piracy is an offense against the law of nations. Treason is defined in the Constitution, but there is no authority delegated to actually punish it.
Arguably subsequent amendments expanded the list of offenses that could be punished as crimes: enslavement, violation of rights by a state actor (but not a federal actor), or deprivation of the privilege of voting on several grounds.
So, a consistent application of originalism makes some determinations easy, if distasteful.