I don't understand how implementing RC pools is a soft fork

I don't think resource credits are actually part of consensus at all, so any changes to it are a soft fork. It's just that all of the witness nodes enforce them and won't let transactions into a block unless the account has enough RC. If you run a witness node and disable resource credits then you should be able to include transactions into a block from accounts that don't have enough RC for those transactions and it will still be considered a valid block and valid transactions. The idea is that if you are doing this then you would presumably lose witness votes and not be able to make many more blocks.