I pretty much agree with everything
@pistolpete11 said but I will add a few other observations.
1. His striking speed was bonkers even by today's standards and you can see that objectively in all his video demonstrations. Whether that would have translated into actual fighting against pro MMA fighters we will never know but his raw physical abilities were legit. His workout regimen was completely insane. He also loved the oblique kick and emphasized it heavily in Jeet Kune decades before Jon Jones began terrorizing people with it.
2. Lee was not completely oblivious to wrestling and grappling. Obviously he wasn't trained in BJJ but he was friends with wrestler Gene LeBell and trained with him.
He literally submits an opponent with an armbar in the opening scene of Enter the Dragon. He showcased striking more in his movies because he felt that's what kept people entertained (sound familiar?) but he was well aware of the importance of grappling and submissions at a time when it would not necessarily have been easy (or expected) for him to learn those techniques.
3. The main importance of Bruce Lee in the MMA context IMO is that he embraced the idea of training and mixing multiple different martial arts during a time when most instructors and practitioners thought that was insane. During his time the idea of "my karate style is better than your tae kwon do style" was the prevailing approach and his willingness to challenge that philosophy was significant. If you took a time machine back to the 60's and showed Lee a current MMA fighter's training schedule with boxing in the morning, grappling in the afternoon and muay thai/striking in the evening he would have been one of the few people in that era who would have thought made perfect sense.