I think you missed the point they were trying to make. They are saying that there will be situations where the government has to buy some product domestically,

I think you missed the point they were trying to make. They are saying that there will be situations where the government has to buy some product domestically, but every private business and citizen imports that same product. The law would cause a domestic manufacturer to exist with the sole purpose of providing that product to the government. Since they won't have any other customers, or any competition, the price will be high.