A blockchain can't be sold, but Steem isn't a blockchain, Steem is just a name, and a name is merely whatever the majority of people associate with it. That is ultimately what was sold here. The vast majority of people know what "Steem" is (both the blockchain and the token) through the lens of steemit.com and the various crypto exchanges on which the token is listed.

If they change the meaning of "STEEM" to mean the new Tron-based token, then that will be what Steem is, and the previous Steem blockchain will end up being known as something else (like Steem classic or whatever).