The lesson says, " This happens in one of these two situations. But, it appears from the examples and the quiz that aunque is followed by the preterite imperfect subjunctive only if both a) an action in the past and b) the information is shared by the speaker and the listener.
I think you may be misreading the lesson as in those two main situations. If you read it again it is saying "This happens in one of these two situations:... " and then it's:
1/ referring to a past action
2/ referring to a future hypothetical situation
Within the past actions then we explain that the information can be considered something already shared. So, yes, it is always when the information is shared that we use the imperfect subjunctive referring to a past action.
I hope this clarified it.
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