Restores the functionality of copying of HiDPI (@2x) images during S3 offload of attachments.

wpos-copy-hidpi-images.php

<?php
/*
Plugin Name: WP Offload S3 - Copy HiDPI Images
Description: Restores the functionality of copying of HiDPI (@2x) images during S3 offload of attachments.
Author: Delicious Brains
Version: 1.0
Author URI: https://deliciousbrains.com/
*/

add_filter( 'as3cf_attachment_file_paths', 'wpos3_hipdi_add_hidpi_file_paths' );

function wpos3_hipdi_add_hidpi_file_paths( $paths ) {
	global $as3cf;

	foreach ( $paths as $path ) {
		$paths[] = $as3cf->apply_file_suffix( $path, '@2x' );
	}

	return $paths;
}

add_filter( 'as3cf_find_replace_url_pairs', 'wpos3_hipdi_add_hidpi_find_replace_url_pairs', 10, 7 );

function wpos3_hipdi_add_hidpi_find_replace_url_pairs( $find_replace_pairs, $file_path, $old_url, $new_url, $meta, $old_filepath, $old_meta ) {
	global $as3cfpro;

	$file_name = basename( $file_path );

	$old_filename = $file_name;
	if ( ! is_null( $old_filepath ) ) {
		$old_filename = basename( $old_filepath );
	}

	// Replace URLs for @2x images used by most HiDPI plugins
	$hidpi_images = array();

	foreach ( $find_replace_pairs as $image ) {
		$hidpi_path = $as3cfpro->apply_file_suffix( $image['old_path'], '@2x' );
		$hidpi_file = basename( $hidpi_path );

		$old_hidpi_file = $hidpi_file;
		if ( ! is_null( $old_filepath ) ) {
			$existing_path  = str_replace( basename( $image['new_url'] ), basename( $image['old_url'] ), $image['old_path'] );
			$old_hidpi_path = $as3cfpro->apply_file_suffix( $existing_path, '@2x' );
			$old_hidpi_file = basename( $old_hidpi_path );
		}

		$hidpi_images[] = array(
			'old_url' => str_replace( $old_filename, $old_hidpi_file, $old_url ),
			'new_url' => str_replace( $file_name, $hidpi_file, $new_url ),
		);
	}

	return array_merge( $find_replace_pairs, $hidpi_images );
}

To install the plugin using only the WordPress admin:

1. click "Download ZIP" on this github page
2. go to Plugins > Add New in the WordPress admin
3. click "Upload Plugin" (top of the page near the title "Add Plugins"
4. upload the zip you downloaded to install the plugin
5. don't forget to activate it on the Plugins page

You definitely don't need Pluginception.

Doesn't work for me with latest version of WP Offload S3