Yes, you misunderstood the story
"Unless I've misunderstood the story, Salesforce owned the code and therefore no employee is allowed disclose or distribute the company's property without permission."
Correct, except when this has been signed off months before.
And, they weren't sacked (so it seems) for actually sharing the code - that will come later. They were sacked for giving the presentation.
The Exec text messaged them 30 mins before the presentation - at a time where their phones were likely already switched off (also remember, this is DEFCON - many people turn their phones off all the time there. It's a good chance you'll be hacked otherwise!), and then used the fact they had given the presentation as an excuse to fire them.
Quite shockingly bad management from a Salesforce Exec - which I predict we will hear more about over the next few weeks, possibly including the sacking of the said Exec.