I guess it underlines just how close Borek is to going Slayer, in the sense of an irreversible mental break.
Part of Mathilde's estimation that Borek is willing to make an ally an oathbreaker is probably that she's had a lot of contact with comparatively radical/practical dwarfs, both Gotri and Belegar.
Because frankly? Smelting the coins is not so different, especially from a dwarven perspective. Is he not breaking the oaths of his ancestors with this? Both Mathilde and Belegar (and we in the thread) would argue that one thing harms no one, and the other does, but dwarfs don't really work on that sort of utilitarian ethical logic where it comes to how they feel (and neither do humans generally), so I imagine many dwarfs would see it as the same sort of crime, different in scale rather than kind. (Including Belegar perhaps).
So Mathilde may have a skewed impression of what Dwarven leaders are willing to do, because the ones she's familiar with skew towards the practical. Of course, Karag Dum was noted for being particularly practical, too. And the rulers of Karak Hirn and KaK also had moments like that she's seen (Not sending help and 'forgetting' about disinheriting his heir.)
They're not stereotypes, but they do have an average type of behavior that differs from that of humans. And what's being described here is definitely possible behavior for a Dwarf, but it isn't expected behavior for a Dwarf.
I think the Karak Hirn situation is a good example (and I can't remember how our friend there is called. I tried to look it up in the Dramatis Persona, and couldn't find it. It bugs me).
For humans, conspiracy and mutual murder over rulership is awful, and certainly outside the norm. But it's not that far outside. Not uniquely so, it probably happens somewhere every generation (even after controlling for the population, since there's way more humans). For dwarfs, it was like Dieter IV, someone almost uniquely awful. (Though that gets heavily skewed by the dwarven reaction to bad things like this. It could happen a lot more and just get suppressed.)
The Chaos Dwarfs show that Dawi can built dysfunctional societies just as well as any other species. For Karaz-Ankor dwarves, it's likely a question of whether they can justify it to themselves. And in Borek's case? I think he can justify a lot.