What's stopping said people from grafting together a few Stack Overflow answers today?

Grafting together stack overflow answers requires knowledge of how your chosen programming language and execution environment work. There is at least some floor of knowledge required to synthesize those answers into a working solution.

Vibe coders as described in the OP can just copy/paste exactly what the LLM spits out with much less knowledge. It’s not the same thing at all IMO.

I don't see a major difference in a 20% working knowledge of a language vs 0% if the output works. Neither are writing provably correct code, and that's often fine! Many of the engineers at my company (not a software company) write terrible terrible code using the Stack Overflow method, but our manufacturing line still runs.

20% knowledge could be enough to fix errors yourself. E.g. this function name has a typo, or this calculation looks wrong, or this code has been repeated verbatim 40 times, maybe it should be a function, or why did the LLM change this totally unrelated area of the code?

If you only take exactly what the LLM spits out you can only verify by running. If it is broken in edge cases you can only prompt the LLM to fix. You then have to run the code again to verify the fix, and with zero understanding it may have broken some other edge case (LLMs do this all the time especially overly eager reasoning models).

> 20% knowledge could be enough to fix errors yourself.

It might be, it might not. My colleagues and I are also guilty of introducing unrelated edge cases, but again, we do fine.

The case you're describing is also fairly pathological. There's nothing stopping a vibe coder from digging into their code and becoming a 20% monster like me.