They may not have been citizens prior to the amendment, but they were undoubtedly subjects. They owed absolutely no allegiance to any foreign nation. Only the U.S.
That's exactly my point -- what made them "undoubtedly subjects"? The fact that they were subject TO the control of U.S. laws? (Subject in the Fourteenth Amendment is used as an adjective, not a noun. It does not say "a subject of" but rather "subject to" -- see Websters 1928 Dictionary; more likely closer to the understanding of most of the framers than Bouvier's, which only defined the noun). If that were the case, then everyone on U.S. soil is subject to the control of U.S. laws (except diplomats).
And what made them owe allegiance to the U.S.? Again -- kidnapping? Please, someone, tell me by what legal mechanism first-generation slaves lost their citizenship of their country of birth in Africa.
If the framers intended this Amendment to apply to slaves and slaves only, then they would have written it that way. Surely it didn't just slip their minds. But they didn't. They said "persons." And that must have been for a reason. Can someone answer that? In all your parsing of the intent of the framers someone somewhere must have said why they didn't write "persons of African Slave descent".