I read some research by the Farrars and Bone, and their conclusion was that Gardner called the people in his coven witches despite them not wanting the label. Apparently, this is one of the reasons he was kicked out of the coven. Also, many other witches who claimed to be hereditary had actually made it up, in order to be accepted by the Wiccan community at large. No one really knows if hereditary witchcraft was real pre-Gardner, though of course, now that the issue has been brought up, some hereditary lines may have been started.
PS. of course, the hereditary witchcraft that Gardner shaped into Wicca is ONE form and ONE form alone of MANY traditions that could be called witchcraft. Just because there is a possibility that Gardner and many others who claimed hereditary roots/contact may have been lying, doesn't mean that ALL modern hereditary witches have Wiccan roots, or anything to do with Wicca at all. As you have already seen, many people could be offended by your statements, so if you are speaking SPECIFICALLY of Gardner-related history of Wicca, you should probably clarify.