You say:
I learnt that the word it can be used to refer a person whenever his/her gender is not known, for example: when someone is knocking the door or on a phone call etc
This is not quite right. First of all, as Andy Bonner mentioned in a comment, you would use either the "generic he"1 or "singular they" when referring to a person of unknown gender; "it" is generally used for inanimate objects and sometimes animals, so when you use "it" as a personal pronoun referring to a human you can sound quite rude.
But more importantly than that, the "it" used in sentences like:
Who is it?
It's the pizza man!
is not being used as a personal pronoun like he/she/they! Instead it is a dummy it or introductory it. From the first link, a definition of a dummy it:
Unlike the ordinary pronoun it, dummy it refers to nothing at all; it simply serves a grammatical function. In other words, dummy it has a grammatical meaning but no lexical meaning.
This YouTuber is using the dummy it in exactly the same way as the examples above. She is not using it as a personal pronoun to "address herself;" she is using it to introduce the person who is speaking (which happens to be herself in this case). To see how "it" is not being used as a personal pronoun, consider this exchange:
Person A: Who is the person who is speaking?
Person B: It is Zoë from the Scratch team who is speaking.
Person B could have used the personal pronoun "she" to replace "Zoë from the Scratch team" and they still would have used the introductory it:
Person B: It is she who is speaking.
1Or the "generic she" as some writers have taken to doing.