It's good abstract advice for writing, but in practice, even when you're writing the first draft of a longer work -- a novel or novella, or even a short story that has multiple scenes -- it's not uncommon to come up with an idea in the current scene you're writing that's going to require changes to an earlier scene. This is true even if you're working from an outline (even if major beats in the story are set before you're writing, you're almost certainly going to have room for improvisation, and that might just lead to better versions the scenes you thought you already had in mind.) And I've also found that when I sit down and discover myself blocked on a story -- or, for that matter, a technical blog post or even documentation -- sometimes going back and doing minor editing what I wrote the day before will kickstart the creative process.