what is the most important in this sentence the present relevance or the fact that this event belongs to the past?

As the sentence begins "it's great that...", whatever follows is the main focus of the sentence.

It doesn't say "it's great that I saw the tallest building in the world!" so the excitement is not around the past event.

It says "it is great to tell people that I have seen the tallest building in the world!" so the speaker is excited that they now get to tell people about this event in the past.