That's the tricky thing with Greek translations of books. They are almost never the same. Another example is "Harry Potter and the Deathly Hallows". In Greek it's Harry Potter and the Relics of Death. (Ο Χάρι Πότερ και οι κλήροι του θανάτου) . I have many other issues with the Greek translations of Harry Potter but let's stick with Duolingo for now ;p
A bit late to this, but... Why is not κλήροι an appropriate translation for hallows? I never saw it as relics but, as the story in the book goes, objects that Death gave to the three brothers, nicely covered in the meanings 1 to 3 given for κλήρος in the wiktionary entry. Not exactly a draw, but certainly closer that relics.
Interestingly most Greeks would probably disagree with you. To them, it's an "s". The slightly "sh-ish" quality is just a result of the way it tends to be pronounced by many Greeks and the intensity ranges from place to place and even person to person. I grew up in Greece and was raised speaking both English and Greek (I'm a half blood :P) and only noticed that my Greek S's have a slight "sh" after moving to Australia and trying to teach Greek words to my friends here. When repeating after me, they kept on coming out with strong SH's where S's should be and I just couldn't work out why. I would insist that Greek simply does not have the sound "SH" and correct them. Eventually, I compared my own pronunciation of S in Greek words with my pronunciation of S in English words and only then did I realise there's a slight difference. My friends were picking up on that and turning it into a full-blown SH. I asked my mother (fully Greek, raised in Greece by Greek parents) about it and she said that it's a family habit. Even other Greeks have remarked at her "sh-iness". So I'm guessing it's just a variant. Point being! Treat all your Σ/σ/ς as S's and don't consciously try to make them SH-y. Depending on the Greeks you interact with, you may or may not acquire it anyway.
Interestingly, this was not always the case. Greek did at one point accept sho as a letter for loanwords, used only rarely and eventually all but forgotten about, which is taken to be pronounced as /ʃ/. Sigma is still taken to be realized as /s/ in Modern Greek, but it does appear to drift, especially in word-final positions.
There is no difference in pronunciation between omicron and omega in modern Greek. The pronunciation of the two formerly distinct sounds has fallen together and is now completely identical.
It's like "ee" and "ea" in English -- "meet" and "meat" now sound completely identical (for almost all speakers), even though they were different several hundred years ago and are still written differently for that reason.