Probably easiest to move it.
But yeah, I see what you mean, Oxana uses the term 'steel your heart' even though the concept and process of making steel is likely very unknown to most metallurgists in all of Savarra, let alone some witch out in the woods. Good catch.
Bronze your heart just doesn't have the ring to it. Also, tbf, it's not like steel is a completely alien concept or anything, I could still see this become a common saying. It could be well-known as a legendary, almost mythical superior alternative to the bronze they make do with in current times. Something you'd tell kids about as the material the weapon of a great ancient hero that lived before the godswar used. I don't think actually needing knowledge of how steel is made is really a factor at all, same way you (as in, a common inhabitant of Savarra) don't actually need to know anything factual about portals or magic in general to understand a story about how they were the things through which wraiths entered the world.
Also also, I don't think he is pointing out that phrase as an issue at all (definitely isn't a bug, I'm pretty sure), only the fucked up text in the second paragraph, lol.