What you say is true but it does seem on this occasion he is claiming the account was hacked.
Who is the user behind the "Side chain" alt-account? He was the one who was after Sportsbet and LFC_Bitcoin and the allegation being made is that he hacked the account belonging to BitcoinGirl.Club
BitcoinGirl.Club should explain what correspondence took place between him and Side chain before the hack so it can be ascertained exactly how the hack took place.
Just because he signs a message it doesn’t prove anything. I could do something bad, claim I was hacked & sign a message from an address owned by me, linked to this account since 2014. What does that prove?