In case anyone’s interested – after some experimenting it seems the example in docs DOES create new comments/database rows and it does do it in the child (comment) model. What I had to do to make it work was explicitly include all table’s columns in the DB::insert statement.
So I used this code instead of the lines in the pink box in the pic below:
return Db::insert('comments', [
'slug' => $this->slug(),
'text' => '',
'user' => '',
]);