Creating a functionality plugin to edit seriously simple podcasting

This is what the ssp_feed_item_itunes_summary filter is for. It lets you change that value through a separate plugin/function. You can read more about filters here.

So rather than making the edit you’ve made, add a filter to ssp_feed_item_itunes_summary:

function wpse_326975_itunes_summary( $itunes_summary, $post_id ) {
    $itunes_summary = get_the_excerpt( $post_id );

    return $itunes_summary;
}
add_filter( 'ssp_feed_item_itunes_summary', 'wpse_326975_itunes_summary', 10, 2 );