Invalidating BPD will lead to disregulation.
In essence you were denying to her ( in her mind) her right to exist. Because to BPD feelings =reality.
This is exactly what happened right before our breakup. We'd had a really close, connected time together, then he had some anger coming up the next couple of days, I didn't know the tools here, and I reacted defensively while he was taking some space. I didn't give him the space to process his feelings, AND I invalidated him, telling him it wasn't okay for him to keep getting angry after we'd get close. OOPS. Little did I know then!
I've learned a ton from this board since then, we have reunited, he has repeatedly said he notices a big difference in me, and he feels safer in this r/s than he ever did. Validate, validate, validate.
To (not) answer your question, I'm not sure how they arrive at that contradiction. I try to put more of my focus on radical acceptance vs. trying to make sense of nonsensical thinking.