can’t modify post title using the_posts filter

You missed the return statement.

function filter_the_posts( $array , $oQuery) {
       ...
       $array[0]->post_title="new title";
       $array[0]->post_content="new content";
       return $array;
    }
   
add_filter( 'the_posts', 'filter_the_posts', 10, 2 );