Well, no, actually you posted that "One of the earliest known mentions" of Messiah ben Joseph was in 250 AD, per the Wikipedia page from which you grabbed your verbiage.
"One of" suggests that, while early, it is not necessarily (and in fact probably isn't, and in fact, actually isn't) the earliest reference to this Messiah, as I showed from my source.
What's funny, though, is that you're trying to argue that the "dual Messianic role" argument is late, but you're doing so from Jewish sources which normally would be predisposed, in the highly-charged polemical atmosphere of the time, to suppressing any suggestion that the Christian interpretation of the Messiah was correct (which the dual-role theory explicitly does). The reason these rabbis wrote about it in the Sukkah was because it appeared to them to be completely reasonable per the Hebrew scriptures. Which is why the Targumists and the Christians before them also drew the same conclusion.
You didn't post a link to your source and the passage you cited didn't mention Massiah ben Joseph or Massiah ben David. There was no date to the commentary. Whereas wikpedia may be wrong on occassion, this article was well sorced with sources cited.