That assumes that a person cannot be granted naturalized citizenship at birth.
It doesn't assume any such thing... it just assumes that that's not what's currently done in the case of born-abroad children of Americans.
[/quote]Picky reading of 14th Amendment[/quote]"All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside." Doesn't say nobody else is (or can be) a citizen of the United States.
Although one wonders why they felt the need to point out that naturalized citizens are citizens - maybe just clarifying that one state's naturalizations are valid in every other state as well?
(But note that the natural-born rights of Americans born abroad - that is, those of future ones - are not constitutionally protected. Congress could just change the relevant acts.)