I'm guessing that you did not actually read his dissent.

Far from agreeing with the Trump administration; his view would effectively end birth tourism but preserve birthright citizenship (and maybe even stronger) for the children of non-citizens, legal or otherwise, who are domiciled in the US.

As for it being cut and dry, do you earnestly believe that "and subject to its jurisdiction" is cut and dry? You might have your view, you might have some historical receipts for it, others have other views, and historical receipts, and others rely on common law readings. There's no single unambiguous interpretation of this phrase. Thomas's dissent is well reasoned and has ample historical antecedents and if adopted would have been the best possible interpretation.