As an aside, I wouldn't categorize the Mosuo as being polyamorous.
They define and organize family very differently The married couple is not the foundation of the society. Instead the maternal line - a mother, her children, her brothers and sisters, her sister's children -forms the basic family unit. There is no marriage as we understand it. How the Mosuo structure romantic/sexual relationships springs out of that fundamental difference.
I would argue that our concepts of monogamy and polyamory are based on the married couple being the foundation of Western 'family'. Without that basis, I would argue that the Mosuo are not 'non-monogamous' as we understand it. Mosuo woman could have just one lover at a time, or her entire life. But that does not make her monogamous - it just means she has one lover at that point. Being non-monogamous requires the idea that sexuality 'should be' confined within a couple if only to disagree with it. After all non-monogamy is basically 'not monogamous'. It is defined by what it is not. Mosuo culture doesn't have that expectation.