In Sefer HaMitzvos, Shoresh 2, the Rambam says that anything derived using Rabbi Yishmael's 13 principles of derivation, or a Ribui (extraneous wording which tells us to include new laws), does not get included in the 613 mitzvos.
I'm wondering why the Rambam singled out Ribui, considering there are many other principles of derivation besides ribui not included in the 13 of Rabbi Yishmael (see Rabbi Eliezer's 32 middos). Different reasons given why Ribui is excluded from the 13, but why did the Rambam specify Ribui? Why not say "or any other principle of derivation"? For example, a hekesh is also not included in the 13 principles. Why didn't the Rambam say "the 13 principles, or a ribui, or a hekesh", or simply just mention hekesh? What's special about ribui?
He definitely doesn't mean that ribui isn't included, but hekesh is included, as evident from Mishneh Torah Hilchos Ishus 1:2 (see the Nosei Keilim there).
Is it perhaps that there are no mitzvos derived through hekesh, but there are through a ribui, and he has to argue that even so they are not in the 613? Does anyone talk about this?