According to the Bach, the Mishna is doing what we find in many Mishnayos, using a terminology from the Pasuk "v'Asher Yishmau etc" instead of a regular phrase. The Gemara therefore asked what is the usage of Mashmi'im, to which the Gemara essentially replied that the Tana is using a phrase from a Pasuk whose meaning is clearly "Machrizin." The way the Bach quotes the Rosh, the Rosh seems to have understood the question and answer of the Gemara differently than the Bach. The Rosh says that the Pasuk brought by the Gemara is merely a proof that Bnei Yisrael are encouraged to do Mitzvos that they already know they have to do.

What then is the tie-in to the question "What is Mashmi'in" and the answer "Machrizin?" It is possible that the question of the Gemara according to the Rosh is not that we can't figure out the term Mashmi'in, as it has a pretty clear Shoresh and can be easily figured out from the context of the Mishna. Rather the Gemara is asking, to what extent does the Mishna mean that we should "sound out" that Bnei Yisrael should bring their Shekalim? Rav Huna answers that included in Mashmi'in is being Machriz to the extent of what was done in Divrei Hayamim. He is mainly coming to state, as the Rosh explains, that we do not say that we should be complacent that Bnei Yisrael will do what is asked of them, and it is therefore important to loudly encourage Bnei Yisrael to do a Mitzvah which they already know they must perform (see also Likutei Ksav Yad in Mishnayos Zecher Chanoch which quotes this Rosh slightly differently than the Bach).

Kol Tuv,

Yaakov Montrose