I have flashbacks and I do talk about them in therapy.
I think that it's worth talking about because:

- the experience of having a flashback can be distressing ime and so I find it helpful to talk about it with T
- the emotions that go with the flashback are certainly real.

I do get what you're saying about memories and the uncertainty that goes with them. I don't think that talking about flashbacks would make them more or less true, or in my experience it wouldn't pursuade you of something as a result of speaking about it.

I have heard of this thing about "false memories" but I'm sure that nowadays Ts would be aware of these things and would know how to take care not to "pursuade" a client about a particular memory or story.