How to skip the creation of landscape thumbnails for an original portrait image

You were headed in the right direction. There is another filter out there that handles intermediate_image_sizes_advanced. You can add or remove custom sized thumbnails in there. Rather than NOT creating your landscape custom sizes on a Portrait upload, maybe try to only create the landscape custom sizes on a landscape upload.

add_filter( 'intermediate_image_sizes_advanced', function( $sizes, $metadata ) {
  // Let's make sure we have the meta data we need before proceeding
  if(!empty($metadata['width']) && !empty($metadata['height'])){
    // Now let's make sure we have a Landscape being uploaded
    if($metadata['width'] > $metadata['height']){
      $sizes['home-slide-medium'] = array(
        'width' => 1000,
        'height' => 504,
        'crop' => true
      $sizes['home-slide-sm'] = array(
        'width' => 500,
        'height' => 252,
        'crop' => tru
      $sizes['video-poster'] = array(
        'width' => 780,
        'height' => 512,
        'crop' =>false

  return $sizes;
}, 10, 2);

I tested it a few times on my end, but let me know whatcha think!!