She told you that yes, she was formally diagnosed with ptsd. End of story! Her doc knows more than you do, both about PTSD criteria and your friends experiences.

She then continued to answer your barrage of questions, further explaining how it affects her. You continue to dismiss it. She shows frustration through pointing out to you that you use something in your past that may or may not have happened as an example of how that still affects you even though you aren't sure it happened. She was trying to show a parallel of how you don't want to be questioned and just get supported unconditionally, but that she isn't worthy of your support even when she has answered questions.

I don't see how it's confusing. She has PTSD. She was diagnosed. It's not your role to tell her that you don't believe her.

The most confusing bit from your post is wondering about how and what you are confused about. She was diagnosed by a professional and told she has PTSD. What is confusing for you about that?