It won't let you select a US address while that message is there.
You just managed to order it when the limitation wasn't there.
I don't have the screenshots to prove it unfortunately, but it actually did. It had the message there in bold red letters and it let me use a U.S. address when I ordered. ¯\_(ツ
_/¯
edit - in fact, it lets me pre-order more and select my U.S. address even though it has the warning.
check it out.
double edit - WOAH WTF. It was literally just in stock via amazon. i just tried it and it worked and I deleted it from my cart before finishing, now it's only available 3rd party lol. oh well who knows.
triple edit - okay i dont know what's happening with that link. when it comes from here it only shows 3rd party sellers, but when i search on amazon uk it comes up as available from amazon. no idea why. anyways, what i said about NO U.S. but then it being totally fine with it still stands.