It's true that Terrence Howard was invited to Oxford Union which is a student-lead debating society, not the University itself. He also spoke at Morgan State University during the event of "Day of Conversation." In both cases he was recognised officially as a celebrity and an actor solely like many other guest speakers before him and not as a credentialed academic nor a scientist. Hence, whatever he said had no official impact on the academic world.
As for the "debunking" of his said theory. Neil DeGrasse Tyson has already done that in a YouTube video with over 13 million views. To be clear, Terrence Howard doesn't have a theory, but a hypothesis, it'll only become a theory after peer review and validation ion by experiments.
Neil deGrase "peer 'reviewed'" his "hypothesis" and effectively showed it to be invalid. You should check it out.
He's also appeared in many podcasts with experts, including the Joe Rogan Experience, they've clearly exhibited that he has no idea what he's talking about. His personal, self-proclaimed theory has been widely and relentlessly criticised, labelled invalid by experts in their fields. In all of honest, sadly, "they" are mocking and treating him as a mere joke.
It's true that some "historical discoverers" weren't academics. However, when emphasising the word "historical," you'll realize that time is a significant factor in regard to discoveries. Back then, discovery was of "fundamentals," for it was a starting point, a new age, a low hanging fruit as said by researchers. Today, however, where most fields have already been established upon centuries of complex intertwined knowledge. It's almost entirely impossible for a single individual to come up with a new unknown. You'll have to be knowledgeable in a vast number of subjects. It's the reason you don't hear of a new groundbreaking discovery by some lone genius. Collaboration with thousands of experts from different fields is the only way forward.
This is one of the fundamental reasons why everyone is laughing at Terrence Howard.