It’s not a rigorous nor formal test of quality, but a simple indicator of representation. Per wikipedia,

A work of fiction passing or failing the test does not necessarily indicate the overall representation of women in the work. Instead, the test is used as an indicator of the active presence (or lack thereof) of women in fiction, and to call attention to gender inequality in fiction.

It wasn’t ever intended to be a mainstay of lit crit, but it’s turned out to be an extremely useful yet quick and simple test, so it’s stuck around in the toolkit.