I assumed he was busy killing a few swarms that escaped (kill it with fire!!!) and they locked him out as there was no time to wait for him. In the trailer it looks like there is red slime in the crack of the door, like SIVA realized they were locking it in, dove at the door, and just barely got crushed inside the crack. Waiting for Saladin would have let SIVA go free. What really made me wonder was why Jolder stayed inside the door. She had plenty of time to step through as it was closing, then we would at least have two iron lords. I like your idea of her knowing she was already infected, so she couldn't leave. My last question is why Saladin tried so hard to be locked in with them.