Most of Bruce's skill...beyond what he knew of WC...was because of his training ethic and conditioning. It is a known fact that Bruce had only completed SLT level and had only just learned the Chum Kui form before he decided 'traditional methods were too fixed' and decided to do his own thing. There were plenty of guys in the original Ip man group out of Hong Kong in 1951 that knew Bruce personally. My Sigung, Woo Fai Ching was one.
In truth, Yip man didnt even train Bruce as much as is popularized. Wong Shun Lung did most of Bruce's teaching although Bruce did spend time with Yip Man. The phenomena of Bruce's skills say more about WC than it does about him. Just think if he had finished the system how skilled would he be?
Like a lot of less dedicated Chunnas, Bruce didnt have the patience for 'traditional systems' and when he didnt have the time to devote to true understanding he called it useless and tried to do his own 'Jeet Kune Do'. This did not sit well with Yip Man at all. In the end even Bruce denounced his creation of JKD. Why? Because it was just incomplete Wing Chun. Good as he was, his understanding never actually went beyond what he had learned in Hong Kong. So of course he filled in the holes with whatever else he could find usefull.
I love Bruce and his movies, but I have LEARNED to love Wing Chun more.