That passage prevents the feds from interfereing in employment contracts through regulation, it doesn’t give them the power to do so.
You have just cited the passage I was referencing in my initial post about it being unconstitutional for the federal government to pass labor laws.
The passage prohibits it to the states, but the Feds need not be prohibited since they are only permitted to do that which is ennumerated in the constitution, and this definitely is not.