>Importantly, HF23, did not actually fork the Steem blockchain into two separate chains, it simply changed the operating code of the existing Steem blockchain.

This is a very important distinction, and it is also false.  
Every code change is technically a fork.  
No one is "forced" to use the new code; the code is open-source. 
Just because no one is running the old code doesn't mean a fork didn't happen. 

If you try to make the argument to say that this is theft, 
you can certainly use the same logic to say the Hive fork is theft.  

That being said, what Steem is doing LOOKS a lot more like theft and securities fraud from the outside looking in, so I could certainly see them winning in court while Hive would not be targeted.  However, it would set a very dangerous precedent; one that puts Hive (and all forks) in danger once the real world actually realizes how this all works.