The "underlying principles"on which these actions are based include several guidelines on time period and locale. BattleMaster is inherently based on the real world in which we live. A specific time period in history, and a specific location on the earth. That is the framework in which we play.
But it does not in any way state that our characters
are in that time and place on Earth, nor that they ever
were there or have any way of referencing it.
The milieu is simply supposed to have the same
atmosphere.
If our characters experience a BT invasion, then
by the principles of SMA, our characters should be expected to react to the events of that invasion, and remember them, and think about them later—not simply forget the invasion happened because it wasn't the kind of thing that would happen in medieval Europe.