I personally don't think MMA is better than Sun Lu Tang. I think that training and conditioning methods in the intervening years have gotten much, much more sophisticated. I think that the union of martial arts represented by MMA and the concurrent cross-training give the fighter a much more varied arsenal to use. Couple that with better training, conditioning, diet, medical care, living conditions, etc. etc. etc. Do you get my point?
I have no doubt that someone who has devoted his life to xingyi training at a very deep level, or boxing, or Muay Thai, or (insert favorite MA here, but avoid Wing Chun) could knock out B.J. Penn. But, B.J. has the advantage of everything I mentioned above and very few of the disadvantages that Sun Lu Tang had to face........... like poverty, for instance.