API page: https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Entity%21...
> array|bool The table field mapping for the given table or FALSE if not available.
That leads you to believe that the return is the same as what you get from $storage->getTableMapping()->getAllColumns($table). But it's not -- the array is flipped, so keys are field names and values are just incrementing integers.