I believe average stats for programmer productivity of production-quality, debugged and maybe reusable code are pretty low - around 100 LOC/day, although it's easy to hit 1000 LOC/day or more when building throwaway prototypes/etc.
The difference between productivity in terms of production quality code and hacking/prototyping is because of the quality aspect, and for most competent/decent programmers coding something themselves is going to produce better quality code, that they understand, than copying something from substack or an LLM. The amount of time it'd take to analyze the copied code for correctness, lack of vulnerabilities, or even just decent design for future maintainability (much more of a factor in terms of total lifetime software cost than writing the code in the first place) would seem to swamp any time gained in not having to write the code yourself (which is basically the easiest and least time consuming part of any non-trivial software project).
I can see the use of LLMs in some learning scenarios, or for cases when writing throwaway code where quality is unimportant, but for production code I think we're still a long way from the point where the output of an LLM is going to be developer-level and doesn't need to be scrutinized/corrected to such a degree that the speed benefit of using it is completely lost!