Per the video tag guidelines, "The video field helps us differentiate types of content that aren't truly movies including but not limited to: official compilations, best of, filmed sport events, music concerts, plays or stand-up show, fitness video, health video, live movie theater events (art, music), and how-to DVDs."

UFC fights, MMA and boxing fall under this category, so the video tag is correct. I'm also not too sure how much Letterboxd would want boxing matches on their site.