Then I guess that they used the word "Feuerer" as a unique term for Ardent which in itself is also a unique term. And I am also native in german. Can you really name one occasion anyone would say "Feuerer" except when talking about the Stormlight Archives?
Oder um mal kurz Klatext zu reden: Als Übersetzung von einer Bezeichnung die in der Realität nicht existiert (ausser eben als Name) ist Feuerer sicherlich ok bis gut. Wobei so etwas eigene Probleme mitbringen kann, siehe Sirius Schwarz in der Erstauflage von Harry Potter.
Edit: @Shaukan-son-Hasweth: I owe you an apology. I misunderstood your first post, I thought you meant that the german translation for the word ardent to be the same as the translation for the title. In retrospect it seems obvious what you meant but I encountered enough ppl online who thought they were perfect in german (and really were not) that I guessed wrong. Sorry.
Anyway, I do think the translator (while surely doing the best they could in an extremely limited time) did not find a real adequate translation for the title. As you already said, the meaning is there, but not quite and as the english word ardent stands more for "glowing" than "putting someting on fire". But again, I do not know what word I would have used. I probably wouldn't have translated the title at all.
To the question you posed, I would always have guessed that Ardent comes more from "an inner fire" than the burning of prayers. Other people burn prayers too and ritualistic burnings are a part of so many religions that I would have not thought the name to be derived of that. But that is all just imo.