While there's something to what you're saying here the meaning of "tend to the needs of their flock" can get really variable in Warhammer. Consider that all the Knights of Morr- all of them- are considered priests. They can't even talk due to their vow of silence, and the "needs of their flock" that they tend to consist largely of smiting the undead and keeping graveyards from being defiled. Likewise the Grandmaster of the knightly order of Ulricans here is considered a priest of Ulric but he's a knight commander and spends his time and energy on battle, not on attending to Ulrican religious ceremonies- he just does that on the side.
This kind of "priest but spends their life on their job, not on their religion" situation is commonplace and Mathilde fits into it really well. The clergy of Ranald is informal and highly variable. While you're right about the recognition of one's peers (or god directly, in the absence of peers, but Ranald hasn't expressed his opinion on this) is needed for someone to legitimately be a priest, that is as a practical matter the only thing that Mathilde needs before qualifying as one of Ranald's. I don't think it's particularly likely that she'll pursue the course, but it would take 1-2 action slots of talking to his cult members that she knows to learn a few formalities and get rubber-stamped, and she'd get the title.