For general imaging, dd is fine.  But you need to read the context here.
  He wants to make an image of the current filesystem in order to write
it back to a thin LV.  For that purpose, you only want to be writing the
used blocks.  Otherwise, you will be wasting all the space and the thin
LV becomes useless.  I suppose if there is enough space available, you
could run a trim afterwards to get the space back, but that's really
inefficient.