The specific phrase "unopposed oestrogen" relates to a risk factor for endometrial (and other) cancers,
but the poster is right in the sense that a high oestrogen/progesterone ratio leads to an increased LH/FSH pulse frequency, which in turn leads to PCOS and anovulation.
But for clarity, don't use the phrase "unopposed oestrogen" for PCOS. Just use it as a cancer risk factor.