600 seems high, but in actually fight to death with actual weapons I would expect more than half to die eventually. That is being inflicted with wounds that result death soon after the other person or then just infections.

Trauma medicine and medicine in general was non-existent compared to today. So any open cuts or large amount of bleeding can easily kill person.

Despite how the article makes it appear, gladiatorial battles were typically not to the death. So while yes, people might die of infection, we don't expect a base rate of half of them dying.