is_home() refers to the Posts page regardless of whether it is on the front page or an inner page.
is_posts_page refers to Posts page only when it is an inner page.
As such the if conditionals in the two code blocks at https://codex.wordpress.org/Making_Custom_Queries_using_Offset_and_Pagination#Offset_.26_Manual_Pagination should ideally be like this:
if ( ! $query->is_posts_page ) {
should be
if ( ! $query->is_home() ) {
and
if ( $query->is_posts_page ) {
should be
if ( $query->is_home() ) {