I want to know, is there any rule/specification for official dead clans which can make them get back without being unofficial again first.
That probably was a bad description so, example.
[Xp] died like a year ago. They were an unofficial clan but everybody left.
They came back a while ago, but immediately got their board back and did not need to become unofficial again.

I'm also in an official clan that has died before, but we are unofficial now, so I want to know, what's the difference why Xp is official and we aren't.

I find that the process in general is confusing. And this thread doesn't exactly help clarify.

As Gentleman said, this revival system is really confusing and it's not explained properly in the link he posted as well.
Maybe take it with a clan mod I suppose.

According to the guide, we already should've been official since a lot of the old members are in our clan. You better bring it to the clan mods m8