I don't see this as a criticism of the HN comments at all, but as just an excuse to restate that a business (or even just a product) is much more than code.
That said, there can be some competitive advantage due to code, and businesses and products vary in how important the code is. eg Google's pagerank was important (perhaps all important algorithms are only valuable if they are patented... because otherwise they just aren't that hard to copy). For a web2.0 site like SO, I think the code itself is fairly minor, and even the user experience (UI layout, interactions and features), though much more work, is also not that hard to copy. But to copy the business is hard.
The most important competitive advantages for SO are network effects (and potential for further growth); and that people are continuously working on it to improve it.
EDIT this makes me realize why an academic at uni doubted how much of google's success was due to the algorithm - I now think he was reacting to people in that environment who tend to give too much emphasis to code and to algorithms. In my view, it's clear that a better way of doing things (such as a new algorithm) can be the basis of a product or business. Of course it can. But it's not enough on its own.