Article I.8 includes the "necessary and proper" clause, which Clarence Thomas neglects to mention. Not that the "commerce clause" isn't stretched well beyond the point of absurdity, but there is at least some justification.
From the same opinion in Lopez vs US:
In addition to its powers under the Commerce Clause, Congress has the authority to enact such laws as are necessary and proper to carry into execution its power to regulate commerce among the several States. U. S. Const., Art. I, 8, cl. 18. But on this Court's understanding of congressional power under these two Clauses, many of Congress' other enumerated powers under Art. I, 8 are wholly superfluous. After all, if Congress may regulate all matters that substantially affect commerce, there is no need for the Constitution to specify that Congress may enact bankruptcy laws, cl. 4, or coin money and fix the standard of weights and measures, cl. 5, or punish counterfeiters of United States coin and securities, cl. 6. Likewise, Congress would not need the separate authority to establish post offices and post roads, cl. 7, or to grant patents and copyrights, cl. 8, or to punish Piracies and Felonies committed on the high Seas, cl. 10.