Read carefully , they only thing which indicate the flat design does is mask internal hot engine part which only will be visible if you are behind the aircraft
https://aviation.stackexchange.com/questions/21081/advantages-of-square-over-circular-engine-nozzle
For decreasing IR signature the length of the nozzles have to be longer to allow the cool air to mix. For F 22A it is mainly for rcs.
What I understand is that since the angle at which we look at the target changes so there will be a change in RCS and with that change we can track the target
@StealthFlanker Can you explain the concept
Thanks in advance