My guess is that he made a quick appearance at the party simply so he could get his last word in to these folks. I also suspect that the third person was outside and called or texted them as soon as cops started to respond, allowing them to make a clean escape.
The cops just happened to be on a training exercise nearby and responded quickly... had that not been the case, the shooters would have had several more minutes to do their thing before having to bug out.
Farook probably made an announcement that all at the party had to declare themselves for Allah.
When they didn’t, he went out, armed himself and came back and killed the infidels.
That is what a real jihadi would do.