Anybody not holding high public office is "unimpeachable".
An "absolutely credible, scrupulously honest" source is something else altogether.
actually impeachable also goes to credibility or criminality, both may be applicable here:
1 a: to bring an accusation against b : to charge with a crime or misdemeanor; specifically : to charge (a public official) before a competent tribunal with misconduct in office
2 : to cast doubt on; especially : to challenge the credibility or validity of impeach the testimony of a witness 3
3 : to remove from office especially for misconduct