It applies equally to non-U.S. nationals. The U.S. just ignores the requirement because those people don't have standing to sue in U.S. courts, and the U.S. has the de facto ability to ignore rulings by other courts.

The Constitution tends not to refer to "nationals" or "citizens" but to "people", as the general intent was to limit the things the government was allowed to do, not to whom they could do those things. The likely argument then, as it would be now, is that if the U.S. can blanket-surveil a foreigner, they could easily convert those same capabilities to a U.S. national.