Although in daily speech English people may say "No, it is not me/her/him", Duo is wrong in disqualifying answers such as "No, it is not I/she/he", as these are perfectly correct. I have reported it several times now, and expect my report will be answered eventually, but until then I'd like to draw attention to it here.
A: Hvem er det i det her billede? Er det dig? (Who is that in this pictue? Is it you?)
B: Nej, det er ikke mig. Jeg ved ikke, hvem det er. (No, that's not me. I don't know who it is)
The point of this isn't really to learn the sentences themselves, but to understand how sentences are formed and the words in them
I'm sure that you taunted him with ‘He who smelt it dealt it.’!
ObLanguage: Growing up in the USA, we always used the pronunciation /smeld/ and would never say /smelt/, except that we always said /smelt/ in this rhyme and never seemed to think that there was anything strange about that.