So spearmint is not peppermint. Peppermint (mentha pipereta in this study) inhibits CYP3A4 and CYP1A2. CYP1A2 is in the liver and if that is inhibited then estrogen will route down the CYP1B1 path so the ratio of the two metabolites 2-OHE1 v.s 4-OHE1 would be altered. For the PCOS (renamed to PMOS btw) if they start with elevated 2-OHE1 after the peppermint this could result in improve estrogen signaling because the now raised 4-OHE1 has much higher affinity than the 2-OHE1 for ESR1 (Era). ESR1 in particular is known for lowering blood pressure.

This particular path isn't that crazy, but it would only be for a subset of folk. From the study there is something simpler: menthol acts as a natural calcium-channel blocker, triggering vasodilation to reduce blood pressure.

>I wouldn't expect mint-family herbs to have much hormone modulating effect at all

Mints are closely related to pennyroyal (Mentha pulegium), which was traditionally (and dangerously because of toxicity) used as an abortifacient. Its mechanism of action is still unknown, so hormone modulating effects seem plausible to me.