It's really not that clear. For example there are specific procedural requirements, enforcement cannot be capricious or arbitrary, they can't be vague, etc etc

The Secretary of State imposed well-understood restrictions on two well-identified people, so there's no vagueness.

The underlying executive order authorizing these sanctions was based on specific concerns about the actions of the ICC, including specific arrest warrants against government officials of a country the president believes is a close American ally. So it's not arbitrary and capricious.

Again, I emphasize that whether US law authorizes doing this does not at all answer the question of whether it's a good idea. In a healthy country, Trump would be impeached on the spot for trying to pick this fight, because even people who don't like the ICC would fear discrediting their ideology. But there's no question under US law that these sanctions are valid.