These expressions don't themselves have the same meaning.
Is the sentence above grammatical? Do reflective pronouns have a grammatical case in English? And if so, does the reflexive pronoun in These expressions don't themselves have the same meaning belong to the predicate thus being in the accusative case?
Yes, it's grammatical. e. neither accusative nor nominative.
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Yes, it's grammatical.
Reflexives are 'plain' case, i.e. neither accusative nor nominative. Their forms are unvarying, whatever their function in the clause.
Note that in your example, "themselves" has an emphatic use, where it functions as a modifier in clause structure.