No, that is not true (what I bolded). I had PTSD for several years that was professionally diagnosed and treated, and I did not remember the major parts of the event for years. I still had PTSD. My flashbacks and triggers just made no sense to me; I didn't know what they were or what they related to. I thought I was going crazy, like having psychosis of some sort. At first my shrink thought I was having panic attacks.
After a long time, the rest of the pieces fell into place as the memories came back slowly. My flashbacks and triggers made sense then. It's like they were hinting of the real event.
I was never told that not remembering it was unusual at all. It is definitely still PTSD even if your mind has blocked key parts of the event. Mine was called (a) dissociative disorder/PTSD, and maybe it's not always considered dissociative, though. I'm not sure about that part. Dissociative states can affect memory, absolutely. They aren't considered normal states of consciousness.