I'm not a dev, but to me writing before rendering makes perfect sense. Why would you render a bunch of images when you don't know what's happening and being said in a scene?
There are two kinds of things to write.
One is the story which is just the rough outline of what's going to happen and the other is the dialogue.
Writing the story before doing the rendering is a given but writing the dialogue before doing the rendering is not a good idea.
How are you going to write the dialogue when you don't even now how the renders are going to look like because they are not done yet?
It's simply easier and allows for more creative freedom to create the dialogue for a render than the other way around.