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The English approach here, as with other linguistic matters, is to solve the problem by using more words.

"Juan thinks they are going to promote him, but I'm not so sure."

Spoken English can easily express that meaning without the additional words by putting the stress on either “thinks” or maybe “they”.

Granted this doesn’t address the claim of the original post.

I'm not a native speaker but even for me it's obvious you can just say "naively thinks".