Yours is an excellent question!

We do state on our site that "if a woman's cervix is dilated by a few centimeters early in labor (or earlier in her pregnancy), but she has not had any bleeding, she does not yet become either a niddah or a yoledet."

The distinction is not between the introduction of a foreign object and natural opening. Even if the cervix opens on its own to emit something (as in childbirth), we assume that there must have been some bleeding. This halacha is actually more explicit in the early sources than that regarding insertion of a foreign object.

The case of dilation of a few centimeters prior to labor is distinct from other cases of the uterus opening. Practically, women can walk around slightly dilated for months without labor being imminent. There is no historical halachic record of such women having been considered niddah for months! Additionally, what is being observed at this point is the dilation of the external os, the outer opening of the cervix into the vagina. According to most opinions, such an opening does not have halachic consequences.

Near delivery, when we are more stringent regarding dilation, the cervix flattens out and therefore the opening that is observed (or caused by fingers) is likely to be the internal os, which is the opening from the uterus into the cervix.