Compersion is indeed real and I feel it often. It is NOT the opposite of jealousy nor is it a cure or antidote for jealousy. Jealousy is its own emotion, to be dealt with on its own, just like happiness, or excitement, or sadness are emotions. Compersion is a lovely feeling that tends to be *more present* when you're not feeling jealous most of the time, but it *can* be present at the SAME time as jealousy.
You do not have to feel compersion to be poly, nor do you have to be free of jealousy to be poly. Humans vary in their emotions, and poly people are human.
Compersion is more easily experienced if you're feeling relaxed and happy about yourself. Jealousy is less likely to be experienced under these same circumstances, hence the association between the two. Sort of like how having a full belly and contentment are often found together.