Variable Not Working Inside is_author() Array

You’ve converted $taxonomy_id_list to a comma separated string then shoved that string into an array. Your array now looks like

array(
  "1,2,3,4"
);

That is not going to match any author ID.

You are misunderstanding what implode does. This–1,2,3,4,5,6,7— is a set of integers. If place in the array like this– array(1,2,3,4,5,6,7)— you get an array with 7 arguments. If you implode the IDs and put that into the array you have this– array("1,2,3,4,5,6,7")— an array with one argument.

Just skip this step: $taxonomy_id_list = implode(',', $taxonomy_group_ids); $taxonomy_group_ids, if it implodes the way you say, is already the array you need. There is no need to implode it only to try to reconvert it to an array.