"I said but in a fight"

What does anything Trump says mean, really?

But ok, that probably means that Trump is now asking about the armaments(rockets, guns and the like) of the plane, so as to how it could actually win from the opposing plane. That's just my two cents though.

Trump asked "How good is the plane?" The answer was "It's invisible to radar". Trump said "But [how good is it] in a fight?".

Joe: how good is this new Formula 1 car?
Mechanic: It's very fast in 4th gear.
Joe: But in a race?

Looking for meaning in Trump's speeches is like looking for gold in dog ****. You won't any but will feel incredibly dirty at the end.