Landis only lacks credibility if what he is saying is untrue, but everyone in cycling who can put 1 and 1 together knows that Landis hit the bullseye with his revelations* and that anyone in cycling try to deny the credibility of Landis is trying to save the omerta or is incredibly, gullible, stupid, blind or a combination of all.
*these are only revelations to those who believed in LA's myth and miracle comeback. if you read any of David Walsh's 2 books on this; "LA Confidential" and "From Lance to Landis", these are not revelatory statements from Landis, but widely known procedures in pro-cycling.
Hamilton can only add weight to what Landis has revealed. Credibility can easily be establised by the Feds and as we see they like Floyds credibility so far.