How do polygamists expect the gender ratio to work out, mathematically?

The basic flaw being that in modern time, hunter-gatherer groups have been pushed to the absolute most marginl land, which sharpens competition. In more abundant areas and less populated areas, it it likely different.

I don’t know. Do people in your world all display the same kind of stupidity and lack of reading ability?

There is a group of men of around 20% of all men and typically those on the lowest social rungs of society, which to their distress remain childless and without family because they do not manage to attract a woman at any time during their life, or at least not for long enough to convince her to start a family.

There is not a corresponding group of women of anywhere near the same size, because the women tend to share the same smaller number men. Not simultaneously, but over the span of their life. At any time, there is an equal amount of single men and women, but for men the single group tend to remain the same, whereas for women it does not. The result of this “timeline polygamy” is almost the same as for traditional polygamy when it comes to things like excluded men who do not get to have a family. Sucks, but such is life.