So the robots are building the shell of the tunnel before the material within the tunnel (spoil) is removed.

The robots do not seem specifically involved with removing spoil or even carving it into smaller pieces (at least pieces smaller than the tunnel diameter).

Does this mean building tunnel shells is the hardest part of tunnel building, not breaking up and removing spoil?

Or does this approach somehow make it easier to fragment and remove the spoil since it's now in tunnel-sized chunks?

(Edit: Yes, stabilising the tunnel is the hard part, see excellent and thorough answer by rmccue)