Correct. Only elective deferrals or Roth contributions to a governmental 457(b) plan can be considered for this credit. Elective deferrals or Roth contributions to a nongovernmental 457(b) plan such as one established by a charity do not qualify for this credit.
"Governmental" means that the employer is a governmental entity defined in § 457(e)(1)(A):
(A) a State, political subdivision of a State, and any agency or instrumentality of a State or political subdivision of a State
