Daas, that’s only chumash, and it’s bedieved; the rambam says it’s better not to do it, but if she does, she gets schar. For anything else, the rambam doesn’t say that she gets schar at all in the context of limud hatorah, and it’s in fact assur to teach Torah shrle baal peh.
If she gets schar, it’s obviously a mitzvah. She’s eina metzuvah v’osah, but regardless, it’s a mitzvah (like mitzvah aseh shehazman grama). The Ranbam is not mechalel between Torah shvaal peh and Torah shebichsav in this regard.
The fact that we shouldn’t be teaching them is a separate point which doesn’t negate the first point. The chiluk between Torah shebichsav and Torah shbaal peh is said regarding this din of teaching her.