Except that the Twain came long after the 128k. It is also quite possible that Apple wasn't in the least bit worried about the Twain competing with the Mac. The original IIgs models never did have stellar sales, indeed the IIe seems to have been on Apple's price lists after the IIgs was pulled. So there may have been no reason for the Twain. Even if the Twain was a stellar performer with respect to speed, the lack of native software would hinder its sales.

Apple may have also been concerned about R&D and support costs for two very different platforms or diluting both platforms by maintaining two mutually incompatable hardware and software environments. That could result in both platforms losing sales because noone would know where the future was. (After all, who wants to end up with a dead platform.)