A sequential analysis of Cao Cao's suspected DNA could succeed in identifying him positively if compared with DNA sequences taken from a fairly large sample of consanguine (i.e. being akin with him) people. Only who actually knows about descending from the general after more than 50 generations?
Effectively the entire population of Henan would have to be DNA screened but this would give a high chance of identifying the clan/family tribe of the suspected Cao Cao. Interestingly the entire population of Iceland was already DNA screened years ago for scientific purposes resulting in a highly precise genealogical tree of almost all Icelandians having ever lived since the beginning of settlement on the island 800 years ago. Today technology is already more advanced and cost efficient so China should be capable of a similar research campaign in Henan.
Well, this kind of tests like the one done in Iceland situation was more trying to identify who's related to whom. That's easy. Identifying a single person? That would be difficult. You need a perfect match. You can tell this person belongs to his family, but cannot definitively show this is the man himself. What if the tomb belongs to some Cao's descendent, not himself? We know that many of his descendents were powerful people and could afford a tomb like this.
You are under the impression that we can identify his descendents precisely and definitively. However, this cannot be done since many people could falsely link themselves to Cao since historically, people have the tendency to claim themselves to be descendents of some famous ancient people.
Doing a genetic testing for Henan for the purpose of identifying a tomb is highly waste of money and timein my opinion and should not be done. Also Iceland is a very isolated place, meaning that you can go back and identify everyone. Henan, however, is in the middle of central China, meaning it has been a very fluid place for a long time since Cao's time. It would be extremely difficult to definitively identify Cao's descendents.