εἷς γὰρ Θεός, εἷς καὶ μεσίτης Θεοῦ καὶ ἄνθρωπων ἄνθρωπος Χριστὸς Ἰησοῦς (Scrivener's Textus Receptus 1894)
No article with ἄνθρωπος, yet KJV renders the word with a definite semantic for some reason.
I've since found out there's a question about ἄνθρωπος being the subject or predicate of the construction. So the choice to translate with "the man" seems to be grammatical and not for other reasons. The common strategy is to render the verse in this way: "a human" (ISV), "himself man" (ERV, Weymouth) or "himself human" (NET Bible). However, if ἄνθρωπος is not the subject, then the noun should not be rendered "the man."
Here is the explanation given by The Pulpit Commentary:
Even supposing that the exact construction of the sentence requires "Christ Jesus" to be taken as the subject and "man" as the predicate, the English way of expressing that sense is to say, "the man Christ Jesus." But it is very far from certain that ἄνθρωπος, standing as it does in opposition to Θεός, is not the subject, and must not therefore be rendered "the man."