As much as I'm looking forward to the (eventual) day when SFO is fully connected airside, I do suppose that having 4 different airside zones is useful for containing events like this. I'm sure it wasn't a very pleasant morning for United travelers and folks going out of International G stuck in the lines, but at least folks traveling on other airlines out of other terminals weren't affected.
A few years ago when I was at SEA they had a similar security breach (someone walked back through an exit into the airside area, or something like that). They very quickly blocked key points along the terminal, in effect turning the single air-side area into multiple divided sections so that they could still continue operations in part of the terminal, and only needed to search one or two parts of the terminal.
Unfortunately due to where the incident had occurred there was no way to get from TSA to the United gates, so all flights from that part of the airport were suspended, but I believe other areas were still operational.
Of course this would depend on exactly what the incident was and where it occurred, but it seems that due to the layout of SFO doing something similar would be fairly simple. In fact, they could potentially even use it to their benefit - if T3 was closed due to an incident and they could successfully split it off from T2 and Intl-G then you could push people to those other two terminals to clear security, and once the incident has been cleared simply let them straight back into T3 without the additional delay of going through security.