Maybe I'm the one overthinking it, but to mee. if the FAA wanted to exclude multis entirely from even a multi-rated private (and higher) pilot exercising sport privileges, it seems to be the simplest path to have pulled out part of the pre-existing Part 1 definition of "light-sport aircraft" and simply say:
§ 61.316 What are the performance limits and design requirements for the aircraft that a sport pilot may operate?
(a) If you hold a sport pilot certificate, you may act as pilot in command of an aircraft that, since its original certification, meets the following requirements:
(10) For powered aircraft, a single, reciprocating engine.
I guess we'll just have to see.