This is what I figured too. If they want to build Steem-like smart contracts on Tron blockchain then acquiring Steemit makes no sense. If they want to put that much money and effort they are better off building their very own blockchain social media platform, both will take years to complete. Steem's code is open source too, they can fork it and make custom interface to it without paying a lot of money. They did it with BTFS without buying IPFS. Steemit brand does not have the best rep either so starting fresh new brand makes more sense if they want the social media natively on Tron and/or have full control over it.

Also, PoB is straight out incompatible with any general purpose blockchain which Tron is one of them. Not to mention Steem's capabilty for custom json and other technical superiority like MIRA and Hivemind. Collaboration just makes way more sense, I believe Justin Sun is not that stupid to bring forth lose-lose situation with forced integration.