He isn't one in that he wasn't a proven martial arts fighter. He's considered a really good martial arts philosopher. He was the first to talk about mixing martial arts on a mainstream scale.
Sure, there were already underground "MMA" fights at the time but nobody knew about them. The mainstream world was still caught up in Kung Fu and Karate.
Bruce was philosophising about taking what works for each individual person from each martial art. 99% of the world at the time could not even comprehend what he was trying to say. They just thought Bruce was developing his own martial art.
Even in 1993, when the first UFC event was held, it was billed as, "What was the best martial art?", for the mainstream world because nobody understood the concept of mixing martial arts. Bruce was decades ahead of his time.
It's easy to look back nowadays and say, "It's so obvious that mixing martial arts was the answer," or "I googled and some guys were already fighting mixed martial arts style fights around Bruce's time," but at a time with no Internet, Bruce was the first to bring that thought process to the mainstream world.