Strictly relying on 3rd-party code without understanding the algorithms and complexities can lead to just as many problems. In my opinion you still better be a very good coder when relying on libraries, just plug and play will fail you sooner than later. I have seen my fair share of this.
When requirements slightly change and you have to adapt a library, suddenly you have to deal with 20k (or more) lines of code and have no idea where to even start. You may just need to use a tiny part of that library but the code is so tightly bundled and abstracted (for good reason) that you have to dig through layers and layers just to make a simple change.
You should at least understand the complexities when relying on 3rd-party code, the Go dependency hell of the game "Haunts" comes to mind (but I don't remember the details, so probably not the best example).
A few additional thoughts:
1. I said "plug and play will fail you sooner than later" -- for most startups that "later" never happens or is not of concern. How often has a startup's code-base made into an acquiring company's production systems without a complete rewrite?
2. Also, when reading some of these posts, I tend to look at the author's motivation/background. This is a somewhat controversial claim (on HN) -- so it is bound to stir up some interest/pubilicty. And also, this is probably not the worst way to court some new clients ("hey, we write less code and waste less of your money"). But I do believe that the re-use approach would be well-suited for short-lived marketing campaigns, as opposed to some other software -- think limited scope and fixed requirements. I'd do the same if I had to work on several campaigns that are probably all similar to a degree.