A non-disclosure agreement doesn't prevent people from testifying in response to a subpoena.
What would prevent the other women from testifying in Solis's case is that, as far as I know, none of the other women witnessed Watson assault or otherwise interact with Solis. So what would they testify about? (Testimony that Watson did something bad to people not involved in the case won't be admissible because that's not what the case is about. If for some stupid reason Watson told one of the other women that he assaulted Solis, their testimony about that would be admissible.)
If we’re going there, trying to keep it simple. Federal Rule of Evidence 404(b) Other Crimes, Wrongs, or Acts.
(1) Prohibited Uses. Evidence of any other crime, wrong, or act is not admissible to prove a person’s character in order to show that on a particular occasion the person acted in accordance with the character.
(2) Permitted Uses. This evidence may be admissible for another purpose, such as proving motive, opportunity, intent, preparation, plan, knowledge, identity, absence of mistake, or lack of accident.
Texas is largely the same (I have no idea if this has been interpreted differently)
Prohibited Uses. Evidence of a crime, wrong, or other act is not admissible to prove a person’s character in order to show that on a particular occasion the person acted in accordance with the character.
(2) Permitted Uses; Notice in Criminal Case. This evidence may be admissible for another purpose, such as proving motive, opportunity, intent, preparation, plan, knowledge, identity, absence of mistake, or lack of accident. On timely request by a defendant in a criminal case, the prosecutor must provide reasonable notice before trial that the prosecution intends to introduce such evidence—other than that arising in the same transaction—in its case-in-chief