I've seen him presented as flawed (I'm forgetting where), but I definitely overestimated the extent to which he is actually the horse's mouth, and you're right that others have too. Sexism aside, the general perception may be because we know Wilhelmine was writing when she was estranged from Fritz and had limited access to sources, and often about events for which she was not present, whereas Catt keeps talking in his memoirs about the extensive and detailed diary he kept for events at which he was present. The number of people who've done the "waaait a minute!" line by line comparison between diary and memoirs is probably less than ideal.

ETA: I think there's also the fact that there's a long tradition of pointing out that Wilhelmine (and Pöllnitz) are inaccurate: e.g. Carlyle and Fontane do it, whereas Catt's memoirs weren't published until almost the twentieth century. So I think modern biographers are also copying older biographers with W and P.