the original team is Russian, but they downplay that a lot now (until you buy one and the promo stickers are in Russian). yes, they're registered in Delaware, almost every company that operates in the US is, including foreign companies.
The company makes Flipper Zero as well. When they launched that product, the same topic came up [0]. The strongest argument given seems to hinge on the former nationality/residence of many of the original team. Obviously, different people will weight that above or below the UK address / USA incorporation.