Add this filter to the query.
Due to the nature of fapi, the value and the operator have an unintended level of indirection. You will find them in $this->operator and $this->value respectively.
Overrides FilterPluginBase::query
public function query() {
$this
->ensureMyTable();
$subselect = db_select('comment', 'c');
$subselect
->addField('c', 'cid');
$subselect
->condition('c.uid', $this->value, $this->operator);
$subselect
->where("c.nid = {$this->tableAlias}.nid");
$condition = db_or()
->condition("{$this->tableAlias}.uid", $this->value, $this->operator)
->exists($subselect);
$this->query
->addWhere($this->options['group'], $condition);
}