That is your assumption. I don't see why he would have to be God in order to represent God. Catholic priests are not God simply because they forgive sins in confession. Sins are forgiven when we repent and turn to God for forgiveness. Someone telling you "your sins are forgiven" is stating what God has already done.
Well, the priest is only acting in loco Christi, so the analogy does not completely work. The priest speaks for God because he has been ordained with the power to do so.
This power ultimately coming directly from Jesus. The question is where Jesus derived this power from. I don't think he was just "communicating" forgiveness.
SD