Molex 5039600695: What is the SIM card stopper referring to?
The phrase "card stopper" is being used in two different ways here.

In the top drawing, it's referring to a leaf spring that only gets lifted up out of the way if the beveled corner of the SIM card is in the correct orientation. When a card is inserted incorrectly, it stops the card about halfway in. The small plastic post highlighted in green helps to support the spring while it is blocking the SIM card.

In the second drawing, it's referring to a separate mechanical stop for a correctly-oriented SIM card, which keeps the card from moving out of the connector after it has been correctly installed. This would be part of the product packaging (e.g., some sort of lid or cover).