I would argue that yes this still is debt, because exactly as you said "you want to rewrite it, but you dare not touch it". Businesses using this software are stuck with ten-year-old processes and systems because the cost of changing the systems has become prohibitive. It's true that you'll never have to change it, but that's just because the cost-benefit of making changes is terrible from needing to wade through piles of bad code! So your debt interest payments come in the loss of business flexibility and greater support costs, and eventually in the relegation of the system to "legacy" and the cost of replacement.

But I think you're making a similar good point to the author that some debt is pretty much not actually debt. I'm sure most large systems have throwaway bits where crappy solutions are actually optimal because they are fast, cheap and don't impact core quality.