I'm pretty sure that only the 24 "penthouse" suites at the Manhattan Club trade via II. The main, non-penthouse, units all trade via RCI. So RCI has access to way more inventory than II making trades for the Manhattan Club via RCI way easier than through II.
As for SFX, a Manhattan Club owner needs to deposit a week with SFX for another SFX member to trade into it. If that happens, SFX can pull off a trade, which it apparently did for next summer for someone. FWIW, I had a request for MC via SFX pending for over a year and was skunked.
It is possible to get a Penthouse at the Manhattan Club via an II trade. However, given the limited number of available penthouse units, and the fact that the vast majority of them will not be traded by their owners (who all paid a premium to buy "penthouse" units), it could be that II is essentially telling you that it is extremely unlikely and you shouldn't hold your breath.
I would say the same thing for SFX, or any other non-RCI exchange company.
If you want a reasonable chance of trading into MC, you need to be a member of RCI, or extremely lucky with II or SFX.
-nodge