"One should always start JS apps as early as possible and only delay operation until the "DOM is ready" that want to measure dimensions, etc. (Things relying on CSS)."
I guess the assumption here is that your script tags are placed at the end of the document - because if your scripts are placed in the <head> and they try to do things like getElementById (which does not "rely on CSS") before the DOM has been parsed, you'll definitely get an error.
You can do a lot without accessing the DOM. Assuming you have a real app, and not just a circus of jquery event handlers, then you probably have prototypes to instantiate, perhaps some models to load, etc.
The post is referring back to the old jquery advice of "put everything in $(document).ready".
I guess the assumption here is that your script tags are placed at the end of the document - because if your scripts are placed in the <head> and they try to do things like getElementById (which does not "rely on CSS") before the DOM has been parsed, you'll definitely get an error.