The "rule" you quoted is simply wrong - or, rather, it is a huge oversimplification of a complicated situation. It states that the choice of which form to use depends only on objective facts (is the event complete in the past?), but this is not true. We use the present perfect when a speaker or writer is choosing to present events or situations in the past as having present relevance.
This might mean that the event has only just finished ("I've had my lunch"), or that it has consequences that continue to the present ("I've written that essay"), or that the speaker is framing it within a time interval that continues to the present ("I haven't seen him today", as opposed to "I didn't see him today", which implies that the possibility of seeing him today has ended), or other possibilities.
The "Have you ever ...?" question is an example of this last - it locates the event during a possibly very long window which extends to the present.
Here we have a period extending from the past up to the present, during which Amira might have been to Dartmoor.
The most natural answer adopts that frame, and answers that yes, there were some occasions during that period extending from some time in the past up to the present when she was there, and so she answers with I have been.
As David says, there's the additional issue that the idiom been to (meaning "have gone somewhere and then come back") is only available in perfect form - you cannot say I am to/will be to/was being to Dartmoor, only I have/had/will have been to Dartmoor.
If we change it to in, so avoiding that problem, then the simple past Were you ever in Dartmoor, Yes I was. is possible, but uncommon in British English (I think it is more common in American usage_). Again, this underlines that the choice is in how the speaker is presenting events, not in objective reality.