It's a stupid comment practice where they claim ownership of their comment and place what they think is a binding and effective license against AI using that comment.

It literally does nothing. This is the modern equivalent of making a post on Facebook to assert that you have rights and control of your comments there.

Beyond the tools for editing and deletion you have no such rights in the Fediverse and it's a good way to demonstrate you don't understand how anything works.

Nothing stops anyone or any entity from indexing, ingesting, or scraping federated content.