How on Earth does flying your enemy aircraft over your opponent's airspace give YOU a casus belli.
That defies all reason and logic.
You can't justifiably gain an excuse for war with another civilization by your actions, it has to be their actions.
Otherwise Iran flying a plane over Kuwait is justification for Iran to invade it? Does that make sense.
Regardless of what technology is in the drone, fake or not, what's important is what canbe learned from it in terms of the intelligence it carries, data in its memory banks, the technology of the materials (skin) it is made of, its electronics, software, how it can be used for propaganda, and how high a bid russia and china will place on parts when iran is done with it.