I hope they clarify soon how the destruction will work in SP vs MP (and the scope of MP and destruction-- is it only competitive or are there cooperative elements as well?) They only showed off-screen footage when mentioning the single player, but all of the MP footage with destruction was on-screen; I wonder if that also was a deliberate choice by them?
I don't think the examples you posted show how server-side computations aren't needed for Crackdown. Many games implement some form of destruction-- you could have shown GIFs of ScreamRide and the one cutscene from GTA V as well, but the scale of destruction shown here is on another level entirely. From a technical perspective, the destruction shown here is the gaming application of the proof-of-concept shown at last year's BUILD conference at Microsoft shown
Here:
EDIT: Even though you are responding to a more microscopic showcase of the destruction, it still looks like the level of detail is a step above games on console that don't have this capability. Either way, I think that JaggedSac is saying there how detailed the destruction is even when it is being used on an entire city.