Think about it. The issue made it clear the switch couldn't happen given that the purported SW of that universe was knocked out. Which means...
The Witch we've had since they left that universe to go to robo Earth was the bad Scarlet Witch so the switch had to have happened much earlier than issue 6 but were only being let in on it now.
Exiles Witch presumably had to start acting like Alternate Wanda to not get found out and when the team bashed her head to knock her out she didn't have a chance to tell them about the fake.