Help! Liege Waffle excessively dense
Given the very-specific times and temperatures given in the recipe, I'd bet that deviating at all would probably cause this result.  Step #7 specifically states that you want to slow and limit the yeast's growth, which I would guess is to form very fine gas bubbles in the dough and provide a final burst as the waffle cooks.  Letting it proof for so much longer probably let some of those bubbles merge and escape from the final dough.  In short, don't do that, even if it doesn't rise very much.

If you follow this recipe to the letter and it still doesn't work, check your yeast for freshness.  They may be expired (in more ways than one) and less effective than normal.

Finally, don't beat yourself up if it takes a few tries to get this right.  This is a very finicky recipe - I've never seen so many temps and times in a waffle recipe.  There's probably a reason why the author states that they aren't seen much even in Belgium.