In my understanding, this is only applicable when "an article is being imported into the United States in such quantities or under such circumstances as to threaten or impair the national security". But "national security" is pretextual in this case.

National security is one of the primary motivations for wanting to protect American industry in the first place; it’s hardly pretextual. And even if it was, the law states that whether or not national security is threatened is up to the judgment of the Secretary of Commerce.