Metaboxes in Loop

You use the callback in a class context, but I see no class declaration. Maybe it was left out, but here’s a possible solution without using class.

function rttk_create_boxes() {
    $myarray = $this->rttk_get_posttype_array();
    foreach ( $myarray as $key => $value ) {
        add_meta_box(
            'rttk_' . $key . '_id',
            __( 'Details', 'rara-theme-toolkit-pro' ),
            'rttk_testimonials_metabox_callback',
            null,
            'side',
            'high',
            array( 'key' => $key )
        );
    }
}

// testimonials template form
function rttk_testimonials_metabox_callback( $post, $args ) {
    $key = $args['args']['key'];
    include RTTKPRO_BASE_PATH . '/includes/meta-parts/rttk-' . $key . '-template.php';
}