What did the petitioner state in the I-129F? For Part 3 section 2 only arrests or convictions are covered, so I assume he answered No there? Even item 4.a states "arrested, cited, charged, indicted, convicted, fined or imprisoned".
It is unclear what you mean by "falsely accused". If there was a formal charge I believe a prosecutor would have filed those charges and there would be a record. If it was just a matter of someone stating to police that "Person X did Y" and there was no charge filed I don't believe you have to answer yes to that question.