Converting array of entries to an array of ids
Simply append .ids() to your initial query...

{% set authorIds = craft.users.search('fullName:*' ~ query ~ '*').ids() %}


Without the .ids() method, you're actually working with an Element Criteria Model.

The behavior that you're seeing is the default... it's as if you had appended .find() to your initial query.


  [.find()] will be called automatically as soon as the ElementCriteriaModel is treated like an array (that is, as soon as you...  start looping through the elements)


So instead of allowing the default behavior to kick in, just apply .ids() to the initial query, and get a batch of entry IDs instead of a full array of entries