He was a self made master. In other words, he started out with Wing Chun and trained in it quite a bit, but never learned the whole style, so no one considers him to be a master of Wing Chun. He trained somewhat in other Chinese styles, but never to the level of instructor.
So, was he a master? His students considered him to be so, and there is that. But actually, in Hong Kong where he came from the title "Master" is very fluid, even casual. When I was there in 2002 to teach some stuff and preach in a big demonstration, they called me "Master" without ever asking me for credentials!
One more note. In China, lineage is more important than titles. Bruce Lee, though he never learned the whole Wing Chun style, had a great lineage through his instructor, Yip Man. But then so do I, but I'm not a master of Wing Chun (Yip Man to Lo Man Kam to Kaisai to Professor Cravens to me)