The Rambam (Edus 20:2) and the Meiri in a number of places on 2a, 3a use the same term as Rashi. The Ritva on 5b writes that we actually do not learn this Halachah from l'Achiv -- as is evident from the Gemara there, which tells us that the Edim are not killed because of Ein Onshin Min ha'Din, and not because of a Mi'ut. (The Mishnah writes l'Achiv to make it look like a Derasha since the Tzedukim that they were addressing would accept that more readily than the logic of Ein Onshin Min ha'Din, see Meiri; the Meiri in Bava Kama 55b seems to be taking the Derasha of the Mishnah at face value, but his words can be learned in a manner consistent with the Ritva as well.)

This raises another problem; if Ein Onshin Min ha'Din is the reason Edim are Patur by Ka'asher Asah, why do the Rishonim write "v'Lo Ka'asher Asah?"

The answer is that there are two steps. First, we must determine that the Torah only addresses a case of Zamam and not Asah. That is "v'Lo Ka'asher Asah" that the Rishonim write. Then, we must ask why do we not automatically assume that the Torah wants us to give the same punishment by Asah, even though it is not written explicitly. To answer that, the Gemara proposes that Ein Onshin Min ha'Din.

M. Kornfeld