I think Pat is intentionally trying to mislead here.
He implies this was part of the Constitution. In actual fact various Acts of Congress had this requirement to become naturalized. The prohibition did not apply to those who were native-born. Dred Scott attempted to proclaim that blacks weren't citizens, ignoring legal and political history, but that decision was rather violently overturned.
The "free white" requirements went away after the Civil War, even for naturalization, except that restrictions on naturalization continued for Asian immigrants. Even for Asians, the restrictions didn't apply to those born here.
(1) Buchanan is smart enough to know that the Naturalization Act of 1790 was passed by the First Congress. That was actually only the first example of Congress restricting American citizenship to “free white persons.”
(2) Blacks didn’t gain American citizenship until the 14th Amendment in Reconstruction. The Founders didn’t consider blacks to be citizens. That is why it took a constitutional amendment to make that a reality.
(3) Indians gained citizenship in 1924. The Chinese Exclusion Act wasn’t repealed until the Second World War. The citizenship status of Asians and non-Whites was litigated in the Supreme Court down the mid-twentieth century.
(4) Congress banned immigration from Asia, the Middle East, and North Africa in the 1910s and 1920s.
(5) Congress passed the Immigration Act of 1924 specifically to ensure that immigration flows came from traditional sources in Northwestern Europe.
(6) It wasn’t until the McCarran-Walter Act of 1952 and the Immigration Act of 1965 that the floodgates were thrown open to the entire world. Even then, as late as the 1960s, the advocates for the Immigration Act of 1965 felt compelled to promise (more like lie) that it wouldn’t change the ethnic balance of the country.