It is not a horrible assumption for the vast majority of developers.
Even if the business they're supporting is doing something genuinely new (and IMO that's rare), most of the software written for that business is almost certainly going to do something similar to a bunch of existing software.
ChatGPT's Vision add-on is able to do things like generate code for a web dashboard from a screenshot of a mockup because nearly every web dashboard is extremely similar except for the branding and labels on the data/elements.
Most developers working for a business are writing that kind of code. It's the dev equivalent of hiring an architect and contractor to build a house. The result may very well be unique overall in some way, but nearly all of it would be based on well-tested designs and components. Most people wouldn't want the contractor to fabricate custom roofing panels out of recycled Russian submarine titanium, because even though it might have a "wow" exotic factor and some tangible benefits, having maintenance done on it would be a nightmare, and there are likely to be surprises no one considered because it was the first house to use them.
Software is even worse in some ways. I've seen complex products built on flash-in-the-pan frameworks of the moment that ended up with problems so close to the foundation that fixing them would require a complete rewrite or forking the (now unsupported) framework and making low-level architectural changes there. At least with a titanium roof, the worst-case scenario is having to replace the entire roof.
There's some audience bias here, because people who read Hacker News are more likely to be working for startups that do something genuinely new, and maybe even writing code that does something genuinely new, but IMO it's still not very common.