The man was already forgiven by the local ekklesia. Again, Paul is saying if they had forgiven him so did he....and why did he do this? So no advantage could be taken of by Satan.
Whether Paul agreed or not the local ekklesia had forgiven him.
A community can forgive someone for something, I can forgive someone in my family for something, but that does not absolve that sin from their soul. Only an ordained Bishop or Priest can in the sacrament of confession. Unless they make a perfect act of contrition before they die.