The author agrees with you. In the final paragraph, the author says that they allow ++ and -- when not using the value, as such use poses no harm.

Also, you can use a comma with += and -=,

  for (int i=0, j=n; i<j; i+=1, j-=1)

but keep in mind the decrement must usually happen before the loop body (post-increment reverses to pre-decrement).

  for (int i=0, j=n; i<j; i+=1) { j-=1; ... }
  for (int i=0, j=n; i<j; i++) { j--; ... }