Three questions come to mind about the meaning of the word האומר – one who says - in this context.
Rambam moved from belief to acknowledgement. At the beginning of each section of Halachot in MT, Rambam lists all the Mitzvot that underlie the rules that will be discussed in that section and they are supposed to parallel and be traced back to Sefer Hamitzvot. At the beginning of Hilchot Yesodei Hatorah he lists this Mitzvah as לייחדו – literally to make Him unique. What exactly does he mean with that? Wolfson suggests that Rambam is thinking of the Midrash Shir Hashirim 7:11 (and from there this entered into the daily prayer book) ומיחדים שמו שתי פעמים ואומרים שמע ישראל ה' אלוהינו ה' אחד – They declaim His uniqueness twice [daily] by saying Shema … Thus not only is it a requirement that one believe in the uniqueness of God but also one has to declaim acknowledgement of that uniqueness. Halacha takes a theological Mitzvah and turns it into a practical performance; belief becomes a declaration.
Here too when the Mitzvah is listed at the beginning of these halachot Rambam changes it slightly to give it a practical performance format –
Again, believing is changed to” bringing to mind”, a willful act rather than a simple belief. This is further confirmed as the Halacha describes this prohibition (idem 1:6) -
But what does “bringing to mind” entail? In Hilchot Avodah Zara 2:6 the detailed description of how this prohibition is transgressed
Rambam then adds- ואחד העובד עבודה זרה, ואחד המגדף את השם – whether someone worships an idol or curses the Name … Clearly the two prohibitions are similar both in their context and their action so much so that acknowledging more than one god is seen as cursing Him. Considering that the prohibition of cursing God is only transgressed once one declaims the curse
One may therefore assume that acknowledging an idol is done by declamation too. A declarative acknowledgement of an idol as true is the practical transgression of “bringing to mind” that there is more than one God so by extension when Rambam said והאומר שיש שם מנהיג, אבל הם שניים או יתר – a Min is “one who says that there is a Leader, but there are two or more” he is only considered a Min if he says so – if he makes a declarative acknowledgement of a plurality of gods. Thus someone who cannot conceive that God has no substance, that such an entity could “exist”, but maintains that God is unique, however contradictory that position is logically , he is not a Min; he is just a misguided simple unsophisticated Jew.