it should be Dafydd ap Hugh. Ap being the welsh 'son of' equivalent.
looked him up and can only assume he made a mistake when he changed his name to that.
While I'm not a fan of the man's work, I do think this is going too far, especially as he did not make a mistake.
The convention in Welsh patronymics is to use "ap" when the father's name starts with a consonant, and "ab" when it starts with a vowel sound. However, even that's not a hard and fast rule, as there are plenty of exceptions going both ways if you look. H isn't a vowel, but the sound is soft enough that "ab" makes sense. Besides, it's his name, his choice.
As for the book where his name was misspelled Daffyd, all I can say is that one of the reasons I was named David rather than Dafydd is because no one who isn't Welsh can ever seem to spell it correctly on a consistent basis.