It's a useful distinction, but not necessarily at odds with what I'm trying to say. I'm more saying that all code has interest on it. "bad code" is high interest, "good code" has interest that at times is close to zero.
Another response gives the construction analogy (I try to avoid construction analogies for software building, because it's a bad analogy.. but I would digress - story writing & editing I think is a much stronger analogy for software development).
Let's say there exists two townhomes side-by-side (attached building). Let's say you need to replace one. The cost to replace one is a function of its attachments to adjacent buildings and overall build quality. If that other building were not there, all costs associated with compability would be zero. We can draw this analogy much further though, let's say to inspect the electrical lines - you need to remove half of the dry-wall of the existing building that you do not want to even touch. Had the electrical lines been put in correctly, that re-do and re-painting of the interior, the temporary eviction of tenants would all be unnecessary. This is the example of the overhead that comes with code. Anything that is 'attached' to something else is going to be more expensive to do anything with compared to it not being 'attached' to something else. The cost of considering and maintaining that attachment is not related to the value provided by that attachment, but instead to the quality of the attachment.
The 'debt financed' metaphor comes in when instead of doing something right, it's done badly which means anything that is attached to the 'badly done thing', needs to incur a high overhead cost to even just understand the badly done thing (every time, and whether we are even trying to modify it or not - this is high interest code). In comparison, if the system is decoupled and the APIs clean, that other thing might need only a moment of consideration instead (low interest code)
At the end of the day, it is to say - if you are replacing the 11th floor of a building, you have to consider every floor whether you want to or not. That is more expensive than if you did not have to consider those other floors. Considering the impact to 10 other floors is always more expensive than if those floors did not exist at all. The cost of maintaining those floors though is not a fixed cost, not all floors have the same maintenance cost. The maintenance cost of equivalent floors is not even the same, it's a function of the quality of those floors. Even high quality floors can be low-quality if we are trying to do something the designers did not expect. It's very context dependent, but all of which is to say that the value from features is significantly independent of the cost of the thing that delivers those features. The thing that delivers those feature might be very high cost, or it might be low - but it will have a non-zero maintenance cost. Simply reading a list of 10 items is more expensive than reading a list of 5 items. If anything, those extra "5" modules have at least that overhead cost, it's not zero. The debt financing analogy comes in to describe how high that interest rate is. Saying "code" is a cost is essentially just saying that all code has a non-zero maintenance cost (maintenance being the activity of reading, understanding, debugging, modifying, or deleting code).