Get Authenticated User ID in Hook Cron?
There's no such thing - cron always runs as the anonymous user.

This is forced by the drupal_cron_run() function in Drupal 7:

// Force the current user to anonymous to ensure consistent permissions on
// cron runs.
$original_user = $GLOBALS['user'];
$GLOBALS['user'] = drupal_anonymous_user();


and Cron::run in Drupal 8:

// Force the current user to anonymous to ensure consistent permissions on
// cron runs.
$this-&gt;accountSwitcher
  -&gt;switchTo(new AnonymousUserSession());