Are the Laws of Dyvnwal Moelmud genuine?

The standard spelling of the name is probably Dyfnwal Moelmud; the spelling of Welsh names varies a lot.

Dyfnwal Moelmud probably was a real person who lived about the 5th century AD. He was supposed to be a grandson of Coel Hen who probably lived about AD 400.

Only two known historical documents mention Dyfnwal. A tenth-century genealogy in the British Library (Harley MS 3859) identifies him as the grandson of Coel Hen, and ancestor of Morcant Bulc.1 A fifteenth-century genealogy in Jesus College, Oxford (MS 20)[2] also identifies him in the same way.1

https://en.wikipedia.org/wiki/Dyfnwal_Moelmud

And it is perfectly possible for professional genealogist to correctly remember the names and relationships of persons who lived many centuries earlier and many centuries before the genealogies are written down.

The other Dyfnwal Moelmud is not nearly as accepted by historians. He is mentioned in the History of the Kings of the Britons (c. 1135) by Geoffrey of Monmouth. And most modern historians consider almost everything in the book to be inaccurate and think Geoffrey made up translating the book from an old book in the British language.

And a lot of the names of British kings who lived in the centuries BC according to Geoffrey are the same as names in pedigrees of Welsh royal families and living in the early Middle Ages. Thus historians think that Geoffrey took pedigrees of medieval Welsh royalty and used the names as names of kings a millennium earlier. So they consider the earlier Dyfnval Moelmud to be a fake and anachronistic double of the real Dyfnwal Moelmud.

I consider both opinions by historians to be a little bit hasty. I consider it to be much more likely than most historians do that much of the History of the Kings of the Britons is more or less accurate and also much more likely that Geoffrey translated most of it from an older book.

But don't get you hopes up too high.

Geoffrey wrote about AD 1135. And so a book that was only 200 years old might be considered an "ancient" book in his time, "ancient" in the sense of being old. Almost the last line states that the Saxons conquered all of England from the Britons and eventually all of England was united by King Aethelstan (who took the title of "King of the English" about 937). If Geoffrey didn't add that part from other sources, the "ancient book" would date to after AD 927 and so from about 1,500 years, give or take a few centuries, from the era of Geoffrey's Dyfnwal Moelmud. And that would be plenty of time of inaccurate stories to be told about real persons and for imaginary kings to be invented.

And the other meaning of "ancient" -and I don't know if it was used in the Medieval Latin of Geoffrey - is dating from the era of Antiquity. If Antiquity ended and the Middle Ages began about AD 500, a book from Antiquity could be from any time before about AD 500. And possibly Geoffrey merely added the bit about King Aethelstan in AD 927 to a book written centuries before AD 500 and thus when information about a hypothetical earlier Dyfnwal Moelmud was likely to be more accurate.

But the most important part of History of the Kings of the Britons is the part about the reign of King Arthur, and Geoffrey dates Arthur's death in the Battle of Camlann to AD 542, which is just a few years after the date of 537, 538, or 539 given by the earlier Annales Cambriae. So Arthur was pretty much living in the very last decades of Antiquity or the very first decades of the Middle Ages, and would be unlikely to be mentioned in any book written before the very end of the Ancient era. Which would thus make the information about the earlier Dyfnwal Moelmud in the History of the Kings of the Britons over a millennium after his time and thus potentially totally imaginary.