It sounds like STEEM becomes a TRON token because old holders of STEEM will get new Tokens on the Tron Network. It is kind of like Golem and Ethos are Ether tokens but they are still Golem and Ethos. STEEM will be a Tron token but still be called STEEM.
Tron bought BitTorrent (BTT) but there are also independent projects on the Tron platform. I guess that Tron is willing to buy other projects unlike, say, EOS who is just EOS and doesn't own any of the other EOS tokens.
RE: Steemit Joining Tron