Why can't we just parent the "Lid" bone directly to the "base_def_out" bone?

You can Hookflash.

I was going to also set up drivers so the Squash controls could make the different parts of the the chest bulge independently (which requires that you don't parent it to the def_out bone).  But I then decided that it would make the tutorial beyond the reach of a lot of the audience.

So I kept it more simple.