The BTC-BCH split was a fork. If you had bitcoin before, you now had bitcoin on both chains. Blocktrades mentions this will be an airdrop, such that all existing STEEM holders will end up with two tokens (except for Sun).

A swap is distinct in that it requires users to trade or destroy an old token for a new token. That's an interesting model to make sure the new token holders actually care about the project. But ultimately you'd lose the automatic adoption you get from an airdrop