In this footnote, Buffon reported a comment by Bruce on a bird from the
royaume de Juda =
kingdom of Whydah, which was on the W African coast (current Benin, more or less); Bruce had suggested this bird was the same as a bird called
mascalouf or
oiseau de la Croix ("bird of the Cross") in Abyssinia. The former kingdom of Whyda is certainly within the normal range of
Euplectes m. macroura, which is what the plate cited by Buffon appears to show; in Abyssinia these are replaced by
E. (m.) macrocercus (in the nominate form the mantle and shoulders are yellow, in
macrocercus only the shoulders are, the mantle is black). I have no idea what is/was called
mascalouf or
oiseau de la Croix in Abyssinia, or whether Bruce was right or not. What is clear, however, is that this footnote (in Buffon's original French text) was not at all about the "date sparrow", nor about any bird from Gafsa.
Those who appear to have erred, here, are the -- English-speaking only, it seems -- readers/translators of Buffon's text, who misinterpreted this footnote as applying to the "date sparrow". Maybe Latham was one of them, maybe he was misled by someone else, I don't know for sure. In any case, Latham somehow ended up including Abyssinia in the range of his "Capsa Finch" while none of the sources he cited did that; and in the 1793 and 1812 English translations of Buffon's text, the mark referring to this footnote in the text was erroneously moved, such that the footnote was directly associated to the "date sparrow" account title. (FWIW, I've checked a couple of German and Dutch translations, they were all OK -- it really appears to have been "an English thing".)
(So far as I know, no, no
Euplectes occur naturally in the WP.)