I'd translate it as:
I strongly feel that, because there is always someone better than you, I can't particularly say that I'm amazing
I could only say that I'm amazing when I entered the best neigbourhood in the world. Otherwise, I get embarrassed
I saw 「X界隈」 can mean "person who likes X", but that doesn't seem to fit here. However, "neighbourhood" doesn't, either.
That's my main issue, but I also particularly don't understand this part:
Is that 「な」 the one that adds certain emotion at the end of the sentence?
And I don't really understand this part either:
Is that 「の」 the one that adds a tone of explanation?
Is 「本気ですごいでしょ？って言っていいの」 qualifying 「世界」 through 「なんて」? If so, how could I translate it? I understand that the structure 「Aで一番B」 means "the most B among A". So that would turn the translation of the second part of the tweet into something like this:
Only when I entered the best neighbourhood in a world where I can easily say that I'm seriously amazing
I'm definitely wrong somewhere, so please help me.