My guess is that Voldemort would have only wanted to confirm with Ollivander about Harry's wand being his wand's brother.As someone said,Voldemort WOULD have recognized that the effect which took place was the Priori Incantatem,or he would have atleast have had a shrewd idea.He would have only wanted to reconfirm with Ollivander 😕.Even Fortescue went missing along with Ollivander.One cannot overlook the fact that Harry DID spend ALOT of time with Fortescue during his break before starting his third year.So maybe Voldemort is trying to garner as much information about Harry as possible.It does sound implausible and incredibly ludicrous that he would question Fortescue of all people,but perhaps Voldemort doesnt want to draw too much attention to the fact that he is trying to know about Harry and thus putting the others(meaning,those who are close to Harry) on guard 😕