Yes, it is not a myth. It has been scientifically proven that a woman is more fertile after a miscarriage, but only for a short time period.
Usually, the woman is "super fertile" for only around 4 to 6 weeks after miscarrying. After this period, her fertility will return to normal, which does not mean that it decreases.
However, this extra "amount" of fertility does not mean that you should get pregnant straight after a miscarriage, as your body might not be prepared yet.