Nothing is truly and completely new. I'm not formulating my requirements in an extinct language. My point is "filling in the blanks" and "do new things" are a spectrum.

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.