That's true, though I feel that the indicators wrt Mr Gardiner being a highly successful merchant are very clear. And honestly, it's something I see a lot with characters who are opposite counterparts--if one is wrong, the kneejerk assumption that the other must be right, no matter how blatantly unreliable they are.

(I think this is also why people overlook/entirely miss Elizabeth's ambivalence about Darcy after the letter but before Pemberley. She jokes about it herself--a sort of seesaw effect where if one goes down the other must go up. Wickham being despicable makes Darcy a great person--only the point is that it doesn't actually work like that. Darcy being a good person is completely dependent on his own character, not being cosmically yoked to Wickham.)