Oh, no. Poor Twiggy!! How is she doing?
I don't know anything about the rescue in question, and I'm not defending them, but as you and your vets have found out, it's awfully hard to tell a cesarean scar from a spay scar. If the rescue got her as a stray, or if her owners weren't forthcoming, the rescue people, and even their vets, might easily mistake the two.
And often, resources are tight enough that the rescue might not be able to afford a test (say, an ultrasound) on every cat to make sure which type of scar it is.