This is not really possible. My guess is that the government is not willing to spend the necessary quantity of money to get e.g. Amazon or Google to divest of Anthropic and stop providing them computing resources.

How is an unwritten policy about suppliers of suppliers of suppliers going to affect a million companies?

No you don’t understand, they can’t accomplish the same by an informal policy.

Both Google and Amazon are government contractors. With the designation, they might have had to divest their positions in Anthropic and be unable to serve their models.

No informal rule accomplishes that.

How would they implement such a policy? Amazon, Google, etc. aren't realistically going to terminate all business with Anthropic based on an informal policy that the DoD won't write down.