It is and it isn't. The Theban script was not invented by witches and it wasn't created with the purpose of being an alphabet to be specifically used by witches. However over the years it has become popular among witches as a way of keeping private writings secret. It is particularly prevalent among Wiccan witches and a common language to write Books of Shadows or Grimoires in.
From a historical perspective I did a bit of research for you. The Theban alphabet first turns up in 1518 (during the reign of Henry VIII) and was referred to as the Honorian Alphabet after Honorius of Thebes a Magus from the Middle Ages. Some sources do claim it was used as a substitution for the runic alphabet but I can't find anything conclusive on this and to be honest I doubt it.
I think there is a value to the Theban script as quite often a spell does involve having elements written down and it makes sense to have those in a format that isn't accessible to most people. I'm not sure there is much value to the alphabet beyond that, certainly there is no magic or mysticism inherent in it. However it does provide a link to the past and it is possible that another purpose of the Theban script is to link the witch of the modern day to the traditions of her ancestors.
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