since my sound's out on my virtual box I can't actually test, however that menu does use display: none which, unfortunately, is still honoured by screen readers. Since screen reader users aren't going to be :hovering over anything, they'll never get the subs to appear. The reader agrees that their display state is "none" and therefore, they're simply not onscreen and aren't read out. So screen reader users (unless they have one that ignores display: none but the popular readers such as JAWS and Window-Eyes do not) are only going to get to your main-level links.
Pulling the subs offscreen with a ginormous negative margin or just left: -9999px keeps them "onscreen" as far as the reader is concerned, and so is a safer technique.
There are only some unique situations where display: none is ignored, such as labels in a form, but the other situations seem to differ per reader, and are basically bugs or quirks (like JAWS will read a span inside an anchor who is display none if the anchor has a background colour, weeeeeird) and can't be relied upon.