Sorry for the delay in getting to this. After a terrorist attack, even a bombing, some items like fragments of ID, clothing, phones and (sorry) bits of people survive. Certainly dental ID is usually possible too, helped by the fact that bits of bodies remain.
In a major fire which burns unchecked, the heat at its centre(s) can reach over 900 degrees centigrade. At this temperature nothing survives: add in a very large building with lots of contents and it’s a case of finely sifting through vast mountains of ash to find tiny pieces of evidence. Throw in the fact that in some cases the identity of certain people in the building was not known (unofficial sub tenants, some apparently without the right to be in UK, visitors, etc) and you are left with a horrific and painstaking task.
Ultimately they may never formally find evidence of some people who are missing. Don’t let conspiracy theories get in the way of the facts arising from an intense, horribly destructive fire.