At altitude most AA is radar based, thus a lower RCS will help directly in not being seen. So you can get closer to the enemy you still can see because of the higher RCS. For BVR fights this is essential.
Those machines shouldn't be low level though. IR missles don't care about RCS. It is not their natural territory either.
Third that Cessna is not hidden either. From above (i.e. an AWACS) it can be seen very good. From the ground it can hide low and slow. But so can dedicated planes too, which is practised regularly.
At altitude most AA is radar based, thus a lower RCS will help directly in not being seen. So you can get closer to the enemy you still can see because of the higher RCS. For BVR fights this is essential.
Those machines shouldn't be low level though. IR missles don't care about RCS. It is not their natural territory either.
Third that Cessna is not hidden either. From above (i.e. an AWACS) it can be seen very good. From the ground it can hide low and slow. But so can dedicated planes too, which is practised regularly.