The gemara brings R' Yochanan that there are three lo ta'asehs that have malkus even though they don't have an action, a ma'aseh. One of them is temurah. R' Yochanan himself says here in the gemara
לא תתני ומימר, משום דבדיבורו עשה מעשה
"Don't teach temurah, because [there] an action is done with his words" (Rashi - it makes chulin into kodoshim).
I hadn't understood what this really explains: R' Yochanan had three special isurim on his list; why is he excluding this one, and why would it be different? Presumably the others have to be special as well; why else do they carry malkus?
Anyhow, I think Rashi answers this:
לא תיתני מימר־ בכלל שאר לאוין שאין בהן מעשה
"Don't teach temurah - in the category of lav'im that don't have an action."
He's not telling the tana to leave it off his list of three; he's telling him to leave it off a different list somewhere, a list of things that shouldn't have malkus because they have no action. All three here are probably to be left off that list.
לא תתני ומימר, משום דבדיבורו עשה מעשה
"Don't teach temurah, because [there] an action is done with his words" (Rashi - it makes chulin into kodoshim).
I hadn't understood what this really explains: R' Yochanan had three special isurim on his list; why is he excluding this one, and why would it be different? Presumably the others have to be special as well; why else do they carry malkus?
Anyhow, I think Rashi answers this:
לא תיתני מימר־ בכלל שאר לאוין שאין בהן מעשה
"Don't teach temurah - in the category of lav'im that don't have an action."
He's not telling the tana to leave it off his list of three; he's telling him to leave it off a different list somewhere, a list of things that shouldn't have malkus because they have no action. All three here are probably to be left off that list.