If you hold down the Start/Stop button for at least 3 seconds, the engine will stop. If you just accidentally bump it, nothing will happen.
From the owner's manual:
Stopping the engine in an emergency
If you want to stop the engine in an emergency while driving the vehicle,
push and hold the “ENGINE START STOP” switch for more than 3 seconds.
However, do not touch the “ENGINE START STOP” switch while driving
except in an emergency. If the engine stops while the vehicle is being driven,
this could lead to an unexpected accident.