Well if they had just watched ROTS, the Jedi purge would be fresh in their mind. So in that sense, it works. But as a stand alone, I can see what you're saying about it being confusing to a new person. Good point.
It works so perfectly as a bridge between III/IV
It makes both movies even better too.
Tarkin was on that Deebo shyt, no wonder Vader respects him.
But when he leaves the room, Krennic be talking shyt again