. . . and he shall have Power to Grant Reprieves and Pardons for Offenses against the United States, except in Cases of Impeachment.
The guy who just got executed was never charged with "an offense against the United States" (i.e., a Federal crime).
This is why the language in the legal documents drawn up in a criminal proseucution is very specific. In this criminal case, the title of the court action was probably listed as: "The State of Texas v. Mexican Sh!t-Bag Invader" or something along those lines. The Federal government was not a party to the case as either the plaintiff, the defendant or the court of jurisdiction, so it has no standing to interfere in the process unless it assumes one or more of those three roles by entering into its own legal action against the State of Texas in a Federal court.
Last I checked Texas,(as much as some Texans don’t like it) is part of the United States.