I think they kind of mixed up Poly and Harem. Poly as far as I know means like an open relationship either can see other people. Harem is multiple females with a single male.
Generally speaking, polyamory is a pretty broad term that means multiple romantic partners but with the informed consent of everyone involved. In a poly relationship, not everyone even has to be romantically involved with everyone else, though that is the most common form shown in AVNs and the definition they're using here. So technically, "harem" is a subset of polyamory if all the girls know about each other and are fine with it.
In real life, it's more common for some people in a poly group not to be involved with each other romantically. For example, if 1 guy is romantically involved with 2 girls and the girls aren't romantic with each other but know about and are fine with each other, that's technically polyamory (usually called a 'vee' relationship structure and the two girls would be called "metamours" of each other, compared to the 'triad' structure MC/Josy/Maya have). Another example could be a married couple where each person in the marriage has a secondary dedicated girlfriend/boyfriend, but the girlfriend/boyfriend aren't romantic with the other spouse or each other. Same where two married swinging couples start dating each other and form a quad. Or if a triad like MC/Josy/Maya added Sage (even if Sage only is romantic with MC). That's all also polyamory, as long as everyone is informed and consents.
Edit: If you've read this and are wondering what's the difference between an open relationship and polyamory, usually the main difference is that polyamory involves love/romance between multiple partners, whereas open relationships might not and just be sexual in nature. So by definition, polyamory is a subtype of open relationships where the people involved have a romantic and/or emotional connection.