seems to imply that he was "credible". There was a line I picked up near the beginning of this chapter, where it is said Wickham seemed of excellent breeding, far above that of his officer company, and that it was obvious in his appearance.

No doubt this was because of being raised so closely in the company of Mr Darcy senior and his son/daughter. The influence was noticeable in comparison with the other military men, even though they were of a high standard or standing, which is what I think the line I read alluded to.