It’s part of the spec —and I don’t have a strong opinion on it — I’m just informing readers. My best guess is that spec authors don’t want more places that can block page loading or rendering, as they consider it an antipattern.
<script type="module" async> will run module code whenever it’s loaded, which may be before the HTML page is actually finished. However, an inline
async module (with no dependencies) isn’t guaranteed to run immediately (although it might) — even though naïvely it feels like it should be able to.