Other mistake: Black Widow and her sister steal a BMW. Getting away they damage the left rear quarter. A few cuts later they show the car again and it has no damage to the left rear quarter. (00:40:15)
Other mistake: During the opening scene when the family is driving down the road, various gas stations can be seen with prices typical for the era in the low single dollars. But during one brief shot through the rear glass, a modern price of nearly $2.50 is seen.
Answer: I can't attest to if the end credit scene was altered, but I can say that it's been Marvel's M.O. to introduce new characters to foreshadow upcoming projects without really giving them much of an introduction. For example, Fury, Thanos, Captain Marvel, the Wasp. It seems this credit scene was meant to tease the audience for her upcoming appearance in the TV show rather than build on who they've already established her to be.
Bishop73