The text as it survives shows layered composition, differing terminology and doctrine across chapters, and references to events after the death of Shang Yang in 338 BCE, so it cannot be a single work set down by his own hand even if some chapters preserve genuine early material.
What would resolve this A securely dated early manuscript or a chapter-by-chapter philological study establishing which sections predate later Legalist compilers would let scholars separate any genuine Shang Yang material from later additions.
OpenClassical Chinese philology and textual historyStanford Encyclopedia of Philosophy