The AI is evaluated on the ruleset the AI is trained to play on, which is a Go variant designed to be easier for computers to implement.
The fact that the AI might have won if using a different ruleset is completely irrelevant.
The fact that the AI can be adapted to play in other rulesets, and this is frequently done when playing against human players, is irrelevant.
Edit: This is confusing for some people because there are essentially two rule sets with the same name, but Tromp-Taylor rules as commonly implemented for actual play (including by Katago) involves dead stone removal, where as Tromp Taylor rules as defined for Computer Science research doesn't. One might argue that the latter is the "real" Tromp Taylor rules (whatever that means), but at that point it is obvious that you are rules lawyering with the engine authors rather than doing anything that could reasonably be considered adversarial policy research.