Again, you're projecting things on him. In a society of perfect selfish rational actors, his behavior would be perfectly normal. He doesn't have that concept because you don't surrender information for no reason when it would hurt your position. That's not trickery, that's simply being rational. If the girls were rational actors they would keep questioning him until they fully hammered out all the details.
I am not convinced that he can know that the information he did volunteer helps his position and that the information he did not volunteer would hurt his position without an understanding of trickery. His strategy here relies on the girls having a specific impression of the meaning of his words other than what they actually mean. If he understands what he has led them to believe, then he understands trickery.
The goal of causing someone to believe something that is not true (which is distinct from merely keeping secrets) is not generally compatible with not being capable of either lying or trickery. Any strategy to attain that goal is either a lie or a trick. Any strategy to cause someone to act against what you understand to be their own interests is a trick. And if he did not have a reasonably accurate model of what is or is not against their interests then he has nothing to guide him in what information to volunteer and what information to withhold, so your argument fails there too.
Also, perfect selfish rational actors are capable of both lying and trickery. If Kyubey is not capable of one or both of those, he is not a perfect rational actor.
EDIT: I decided to post about this to the Madoka general thread.
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