I know very little about boxing, so I'm really not sure. My thought would be that though both were successful, perhaps Ali was an underdog who became successful

I know very little about boxing, so I'm really not sure. My thought would be that though both were successful, perhaps Ali was an underdog who became successful where Tyson started successful but hit a plateau? I have no idea whether or not that maps to reality, that's just pattern matching. The other possibility is that the author of the article knows the same amount about boxing as I do, and chose an unfortunate analogy.