Unless I’ve missed something in the flow of this, It seems the mentioned WPSE answer is correct.
The filter hook referenced, wp_save_image_editor_file, is applied in the wp_save_image_file() function when dealing with an instance of WP_Image_Editor class; giving access to $filename (trac link for below):
* @param mixed $override Value to return instead of saving. Default null.
312 * @param string $filename Name of the file to be saved.
313 * @param WP_Image_Editor $image WP_Image_Editor instance.
314 * @param string $mime_type Image mime type.
315 * @param int $post_id Post ID.
316 */
317 $saved = apply_filters( 'wp_save_image_editor_file', null, $filename, $image, $mime_type, $post_id );
That function calls class method save() which provides no access.
76 /**
77 * Saves current image to file.
78 *
79 * @since 3.5.0
80 * @access public
81 * @abstract
82 *
83 * @param string $destfilename
84 * @param string $mime_type
85 * @return array|WP_Error {'path'=>string, 'file'=>string, 'width'=>int, 'height'=>int, 'mime-type'=>string}
86 */
87 abstract public function save( $destfilename = null, $mime_type = null );
So while it is not after the save, it is the last moment of the name before being saved, and the filter’s usage is intended to be able to skip that save() call.
Regarding first (null) parameter:
Returning a non-null value will short-circuit the save method, returning that value instead.