Because the real world very rarely matches the precise conditions to which a theorem applies.

Not saying it would be a good bet, and not the theorem isn't a good heuristic, but it's not as clear-cut as you're saying.

Edit: I also clicked on the link and it doesn't claim to be a theorem or have a formal proof, it just gives heuristic arguments against using active management, which, again, good rules of thumb, but not ironclad proof in the sense you meant.