A promise, really? To who?
The US doesn't have the required infrastructure to manufacture 600,000 iPhones per day, every day of the year.
And being able to increase/decrease that number instantly as market conditions change (instantly ramp up for coming holidays, instantly ramp down for for slow periods).

Apple's political game with Trump worked like a charm
Apple CEO Tim Cook and President Donald Trump appeared at the Austin facility where the new Mac Pro is being assembled on Wednesday as the company seeks tariff relief.
Well, here you have it 🤓