They aren't credible, but for other reasons. Remember when covid wasn't airborne and thus masks didn't do anything unless worn by professionals? And border closures wouldn't be necessary and only xenophobes would call for them? And then how they banned corona tests by anyone but the CDC? And the approval delays?

They're playing politics, worry about second-order effects before first-order ones and do 180° turns instead of focusing on the core mission of assessing whether something is a) safe b) likely effective. The same situation happened again with the booster approvals, they dragged their feet again and decided that only those above 65 should be allowed to get them when in practice some international travelers already are forced to get more than two shots due to inconsistent regulations.

More nuanced policy, communicating uncertainty and "currently not recommended but allowed" middle grounds would help their credibility.