Every organization is different but no, I don't think most of them should be writing code. I wouldn't put any kind of quantifier on it. Software development is not just "writing code". Shipping code from ideation to production is more complicated than layman want to make it out to be, especially at these scales. This dystopian "nobody works" fantasy that some people wish for is simply that, a fantasy. I'm willing to bet most, if not all Twitter developers were above average across the industry and were producing work of value.