A while back, Wemo did not need a skill to work with Alexa. Wemo was one of the first smart devices that Alexa supported, even before Philips Hue. Back then, there was no such thing as skills. Even when skills came about, Wemo could work with or without the skill. Some time between then and now, things changed. Native support for Wemo was removed and the skill was required. Perhaps you were caught between that time.
You can try installing the Wemo skill, link it with your Amazon account, and then removing and disable the skill. That might cause Alexa to re-sync w/ any old ghost routines or phantom devices and realize they don't exist any more.