I would suggest a different peshat in the Ritvah and Ramban. I believe that the peshat in the Ritvah and Ramban’s answer is that there are some mitzvos that pertain only to oneself (i.e. donning tefillin) and other mitzvos that one must perform for others. Women are exempt only from mitzvas assei she’hazman grama regarding mitzvos that pertain exclusively to themselves. However, mitzvos that they must perform for others they are obligated in.
The mitzvah that parents must give their son a bris milah is a mitzvah that they must perform for someone else – namely their son. As a result, without the pasuk women would be obligated.
Perhaps we can still apply the Ritvah and Ramban’s answer to the question at hand. I would suggest that the mitzvah of disowning one’s produce is a mitzvah that is done for others as well as for oneself. While we are not claiming to state the reason for the mitzvos, one of the shorashim of this mitzvah is giving one’s produce to others. Whoever wants can come eat from the fruits of another person’s field. Perhaps this renders the mitzvah to disown one’s produce a mitzvah done for others, which women are obligated to keep.