Firstly because of the power dynamics. Anti semitism is of course a problem in the UK but today far right racism is far more dangerous(A Labour MP was murder by a far right extremist and the far right extremist who drove a van into a crowd of muslims, killing one man and was hoping that he might have being able to kill both Cobyn and Sadiq Khan as well). And then we have the state led racism - such as deportations, arms selling etc, it really wasn't that long ago that the government order vans to driven around the capital city asking the public to report illegal immigrates. The Facebook group is essentially anti semitic losers posting anti semitic youtube videos, they have no actual political power.
Secondly while I haven't read the full report(It's very long)in The Guardian article shows no evidence of Corbyn saying anything anti semitic, Corbyn says he was added to the group rather than actively joined it, and again according to Corbyn he only posted when tagged. His posts seem to be similar to his activist work - having a vote on recognising Palestine and potentially having a a doctor from Palestine come over to the UK to give a presentation.
And then finally since Corbyn has being Labour leader has the Labour Party adopted any anti semitic policies, NO. The party has been quick enough to kick out anyone who has been anti semitic(I image there's a unit dedicated to just Ken Livingston).