I'm not sure about the OA :
Manhattan's rule on past perfect states :
1. Past Perfect is used to clarify or emphasize a sequence of past events. The earlier event should somehow have a bearing on the context of the later event.
2. The words "before" and "after" indicate the sequence of events clearly and emphatically enough to make the use of the Past Perfect unnecessary.
Apart from the above there are more other situations mentioned in MGMT guide in which use of past perfect is not required even when two actions have occurred in the past and one before the other.
1. If both the actions are performed by the same subject OR if the time sequence is already obvious then we do not require past perfect.
e.g. Antonio drove to the store and bought some icecream
2. Main clauses linked by AND or BUT do not require the past perfect. The reason is that in such case we are not emphasizing the order or time sequence here.
e.g. Antonio drove to the store and Cristina bought some icecream
Using past perfect requires us to add extra words. So when unnecessary we don't use past perfect. Grammatically it is not incorrect, but GMAT also tests concision.