I can't directly observe Eliezer winning or losing, but I can make (perhaps very weak) inferences about how often he wins/loses given his writing.

As an analogy, I might not have the opportunity to play a given videogame ABC against a given blogger XYZ that I've never met and will never meet. But if I read his blog posts on ABC strategies, and try to apply them when I play ABC, and find that my win-rate vastly improves, I can infer that XYZ also probably wins often (and probably wins more often than I do).