Most likely its training data included countless Space Invaders in various programming languages.
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i have this feeling with LLM's generated react frontend, they all look the same
Models don't emit something they don't know. They remix and rewrite what they know. There's no invention, just recall...
Trivially, humans don't emit something they don't know either. You don't spontaneously figure out Javascript from first principles, you put together your existing knowledge into new shapes.
Nontrivially, LLMs can absolutely produce code for entirely new requirements. I've seen them do it many times. Will it be put together from smaller fragments? Yes, this is called "experience" or if the fragments are small enough, "understanding".
I think most people writing software today are reinventing a wheel, even in corporate environments for internal tools. Everyone wants their own tweak or thinks their idea is unique and nobody wants to share code publicly, so everyone pays programmers to develop buggy bespoke custom versions of the same stuff that's been done 100 times before.
I guess what I'm saying is that your requirements are probably not new, and to the extent they are yes an LLM can fill in the blanks due to its fluency in languages.
LLMs have their limits, but they really can understand and productively contribute to programs that achieve a purpose that no program on the internet has done yet. What they are doing is not interpolation at the highest level. It may be interpolation/extrapolation at a lower level, but this goes for any skill learnt by anyone ever.