No, I think you've misread that. She asked him about the night he came to her apartment, which he (bad cooper) clearly remembered and taunted her about. I think back then in the moment she just assumed it was him at first, as most people do, but later with whatever terrible thing he did, she began to question could that really be the guy I knew, and now in this new meeting she has confirmed it in her own mind as not Dale Cooper.
There's room for interpretation and the next episodes will clear it up but I don't feel like in any way Dale and Diane were romantically involved before 'that night' or that it was the real Dale. It feels like a violation of trust occurred big time.