Not just the US. Crucially, Wuhan and I think also the rest of Hubei pretty much only tested patients who showed lung abnormalities on CT scans, so only the more severe cases were diagnosed. The WHO knows this. In fact, they've been appearing in the pages of the New York Times and anywhere else that'll have them and gushing about how great this Chinese testing methodology is and how everyone should copy it.
They test much more widely but only those with pneumonia are counted as confirmed. But this is a viral pneumonia. So the undercounting is not that much.
