The US GDP can grow while the rest of the worlds GDP slows. It has happened many times in the past.
But with the current level of US debt what would happen if those countries with a decreasing GDP are unable or unwilling to buy our debt? How high would rates have to rise to make them buy again? Maybe that's not necessary, perhaps the Fed could buy them again with....more money made of thin air.
Regardless I do believe President Trump knows more than I do and he's giving a heads up.