This article doesn't clarify why the authors are so surprised about calcium catalyzing this reaction. The Frank-Caro process is based on calcium carbide and is over a hundred years old. Anyone have any insights here?
The article talks about Ca3CrN3H as a catalyst, giving the impression that the three nitrogen atoms in the compound stay put; it is different nitrogen atoms that get pumped into the reactor and leave as ammonia to be spread as fertilizer. The calcium remains in place, merely making the reaction go faster.
Catalysis is always a bit mysterious because the catalyst is unchanged by the reaction as though it wasn't involved at all. I don't think that there is any expectation that one can discover catalysts by taking reactants as a hint. Indeed, the classic platinum catalyst in a catalytic converter uses platinum, a very unreactive metal.