I think that's sound advice. The strategy I had in mind was courting industries that were physically close to me. I wasn't just calling the businesses, I was showing up at their offices to ask for meetings. My hypothesis was that there are probably plenty of profitable businesses nearby that are disconnected from the tech world, so other software companies aren't approaching them.
To clarify with the sheet metal companies, I didn't go in pitching a replacement to their existing tools. I always opened the conversation by asking them if they had processes that frustrated them, cost them money, or if their existing software failed to meet their needs. But I definitely found it hard sometimes when they asked me what I wanted to build for them and my answer was basically, "I don't know..."
Maybe it's just a matter of picking the right spot on the spectrum. It seems like you want businesses that are niche and disconnected enough from mainstream tech that big players aren't courting them as well. But they can't be so disconnected that they won't talk to you, either.