So without that name being stated a dozen times a project like other characters such as Iron Man, Spider-Man or Captain America, it’s not a total mystery how one could be mistaken about such things.
(Even though they probably didn’t know how the show would handle Wanda’s origin story when previous filmed were produced.)
At least she knew enough to stick with a scarlet-esque hue, as opposed to guessing something completely different like Chartreuse Witch or Burnt Sienna Witch.
Now, to give Jarnie Olsen some credit here, it’s been long overdue for the MCU to officially turn Wanda into the actual Scarlet Witch, though I can understand why the concept for WandaVision may have caused Marvel’s creative teams to hold off on introducing the title.