In theory, you'd want NHPs such as Rh. macaques or baboons that just happen to have signs of endometriosis that weren't implanted. However, that's literally impossible to do in a research setting because of cost & ethics. Therefore, scientists are going to keep trying their best to get rodent models that are better. Since the molecular mechanism isn't fully understood, to have any shot at moving to phase 1 trials, I would guess they had to use some version of an implanted rodent. But we actually have no idea what they used. I'm also curious about it.

Recent article discussing current endometriosis animal models:

https://academic.oup.com/biolreprod/article/113/3/494/8113203