I have no idea what the context was for Naval, but if someone walks to you and say "excuse me I hope I am not interrupting," REGARDLESS of your conversation (because you are in a public place), you stop and give them 15 seconds.
For how do you know they are not telling you your car is being towed, or your kid is being taken away, or you dropped your wallet.
If it were Obama who walked to them would they have stopped for 15 seconds? If yes, then what is the difference?
At the very least say with a smile that you are sorry, but this is really not a good time, but they can email you if it is not urgent.