In that introduction,^1 the author of the segments in square brackets^2 is perfectly clear that the statement^3 is false.

I must record one of the very rare errors of Fabricius here. He (Cod. Apocr. N.T.,i, p.965) cites Sixtus Senensis as saying (on the authority of Serapion) that the Manichaeans so prized the Revelation of Stephen as to carry it in the skin of their thighs! This long puzzled me, and I could not find it in Sixtus. But at last I noticed that at the end of the article just preceding Stephanus, Victor Vitensis is quoted to this effect: The Manichaeans so honoured their teacher that they used to have these words inscribed on the skin of their thighs. 'Manichaeus, disciple of Christ Jesus'.

(My emphasis.)

Even this report is inaccurate. The reference to Victor Vitensis is to Victor's Historia persecutionis Africanae provinciae or "history of the Vandal persecution", book 2.2. There Victor refers to a single individual named Clementianus who had "Manichaeus,^4 disciple of Jesus Christ" written^5 on his thigh.

That is the sum total of all the evidence that exists on this subject. For reference the Latin text is here, on page 12 of the PDF.

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^1 Whoever wrote it; no author is named.
^2 Whoever wrote those; no author is named.
^3 Wherever it comes from; there is no hint where this "article just preceding Stephanus" is to be found.
^4 This could be either "Mani" or "a Manichaean", depending on whether you interpret it as a name or an adjective.
^5 Not "inscribed"; the Latin word used is scriptum, which could mean any kind of writing. It could be paint for all we know.