It's a fair interpretation. But I don't like it because the ad actually says metal tools, and has a picture of a hammer. Sure it may be a subtle way to get someone with a talking sword to contact you, but they know enough about Nighblood surely to know that's unlikely. If Nightblood talks to you, you either know he's valuable enough to not answer a newspaper ad, or he's murdered you.
I've seen the idea that they're looking for sentient splinters of Harmony (or Ruin / Preservation). If there are splinters and they have a physical aspect, it'll probably be metal. It has serious issues, but I don't have a better idea.