They aren't selling them to American customers. The same how you see Xiaomi cars being tested in the US despite the ban, Xiaomi just imports them directly and lets select journalists use them, but they aren't actually sold there.
Waymo can probably import them themselves, they just can't act as a dealer for customers in the US.
Waymo can import the base vehicles but none of the connected tech that comes with normal cars in China. Instead, they are specifically adding their own connected tech (after the car is delivered to the USA), so the ban is irrelevant to them.