They could have towed the diggers across interstellar space.
That's probably NOT what they did, but think about it: the huge moon-laser that pushes the solar sail is powerful enough to fling a four kilometre long starship on its way to Alpha Centauri. Yet the Capital Star class ships built with unobtainium are said to be vastly smaller (and thus lighter), which means there's a huge power reserve in their primary power source.
So...they could have strapped those gigantic diggers on a massive cable and towed them across four light years, and then dropped them from orbit using gargantuan ablative heat shields and strap-on retropacks built with Saturn-V sized rockets.
I don't know about you, but the images this brings are nothing short of "whoa, awesome!". A hundred thousand tonne vehicle landing in the jungle on massive pillars of flame, like a beast straight out of Hell. Kills everything within ten miles of the landing site by just firing the retropacks.
