The earliest documented appearance of the Theban script is in the German abbot and cryptographer Johannes Trithemius's Polygraphia, printed in 1518, a serious technical work on cryptography rather than a grimoire in the modern sense. Trithemius attributes the alphabet to a figure he calls Honorius of Thebes, which is the source of the script's alternative name, Honorian, but the attribution rests entirely on Trithemius's own word: no manuscript associated with any historical or legendary Honorius containing this specific script has ever been found, and the alphabet cannot be traced to any source earlier than Trithemius's 1518 publication.
This does not make the alphabet a fraud in the sense of a deliberate forgery aimed at deceiving anyone about its age; sixteenth century cryptographic and magical writers routinely presented newly devised or newly compiled material under the name of an older or more exotic authority, a common convention of the period's literature rather than a unique deception. The honest description is that Theban is a documented sixteenth century substitution cipher whose claimed deeper antiquity is simply unverifiable rather than independently confirmed or refuted.