That's what I thought. Which means the original post makes very little sense- if your <script> tags are just before </body> there's surely no real delay between that an using DOMContentLoaded?
DOMContentLoaded, in all important browsers, does block to wait for CSS. Manipulating the DOM before this event both increases the chance to make changes before first-paint (decreases "jigger" on page load) and gives you the chance to request more resources earlier.
I guess I never really noticed this because my scripts are always included so much later in the document than my CSS.