The statment, as in, "statistically", reffers to stat averages.
Tiering is a completelly different concept that is based on character usability and influence over the game (so those with a higher influence can be restricted to higher tiers)
The sentence has nothing to do with tiering, nor is it saying it is the best type altogether, it is saying it literally has the best stats.
Therefore, to answer the question, the "statistically best" second type would be Steel, followed by Psychic and then Ice. None of this is saying the types are good, rather than just that there are plenty of pokémon with good stats in them. (Ice is a terrible type and it being so high is all alone a result of
Kyurem)
For the actual best types... generally, the pokémon themselves and how they relate to other pokémon is what matters. And even when thinking of the types abstractly, while one can "score" the types by resistances and effectiveness, the relation between the individual types matters more.
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