I think where a lot of people run into problems with the KISS principle is they conflate simple and easy. More often than not, simple is hard. Which is maybe a soft reason why agentic coding, which is all about trying to make every task easy, leads so consistently to complexity and bloat.
And yes, that's also why LLM code is usually (unless driven by someone who really knows what he's doing) full of unnecessary complexity. Because although LLMs can get a much broader "understanding" of a code base than I would have given them credit for a couple years ago, their "understanding" is still shallow. And what you need to truly be able to achieve that hard simplicity (I heard someone refer to it once as "the simplicity on the other side of complexity", e.g. when Kepler figured out that orbits were ellipses, not circles, and that allowed getting rid of epicycles) is deep understanding. Which LLMs are not capable of, hence why their code is full of the kind of needless complexity that one produces when one has only a shallow understanding of the code. (E.g., you in that enterprisey codebase when you weren't going to be given time to reach deep understanding).