They built a list of 2 to the 100000th X's and then asked for its length. There's only a link for explanation of how, but I believe they represent a list as a DAG, so the repeated structure gets encoded compactly.

I've only just glanced at the source, but it seems to be based on the data structure explained on pg 77 of http://www.cs.cmu.edu/~rwh/theses/okasaki.pdf.

It builds a list of 2^100000 'x items (to put it in perspective a petabyte is 2^50) so obviously the list is not realized.

Looks like random list generation using PLT scheme. The long number is the actual list, printed without breaks.

EDIT: apparently not, see below