External consistency vs linearizability
External consistency doesn't have a fixed meaning. In this context, it has the meaning appearing in the very next sentence in the document:


  For any two transactions, $T_1$ and $T_2$ (even if on opposite sides of the globe):
  if $T_2$ starts to commit after $T_1$ finishes committing, then the timestamp for $T_2$ is greater than the timestamp for $T_1$.