What I don't get is how the fired cartridge case wouldn't be found in the chamber in that example. If the slide goes far back enough to eject the case, wouldn't it strip a new round off the magazine and feed it? Or at least mis-feed it?
On the other hand, when the police arrived they retrieved Zimmerman's holstered gun from his person. So he had time to put the gun in any kind of condition. I don't think we can draw any conclusion from the condition of the gun.
You are absolutely correct, this was only based upon the hypothetical proposed by the original author - one possibility was a non-moving slide.
Remember that the spent brass is ejected before the new round is completely stripped from the magazine and positioned for insertion into the chamber. It's a matter of micrometers, I'm sure, but it is possible for it to blow back sufficiently to eject the brass, but not enough to grab the next round. However, I think you are completely right that a more likely result would be a misfeed rather than a nonfeed.
Anyway, that's all just completely hypothetical, and was more just range talk than actually saying anything meaningful about the scene in question.
I'm not a firearms expert, so other may have more knowledge about this topic.