Custom submit handler never gets called
validate and submit handlers are added to the corresponding form element, e.g. $form['actions']['submit']['#submit'][] = 'foo'.  and same for #validate.

Honestly no idea what those methods are used for, never used them. Maybe internally, during form processing.

Also note that the node form currently has multiple submit buttons (save as published, unpublished and so on), so you'd probably need to add your submit to all of those.