Whether Fresnadillo meant to or not, the lesson on display is that the prudent choice for the occupation is to kill whoever doesn't co-operate with it. An over-abundance of compassion only leads to the necessity of greater barbarism in the long run. It's an argument that uncomfortably vindicates the contention that the only thing the Iraqis understand is a stern, unyielding order. That makes sense - provided that you're prepared to accept that Iraqis are bloodthirsty zombies.
Absolutely not. If that's what the film being 'about' Iraq meant then 28 weeks later is not 'about' Iraq, in that 'the infected should have no human rights' does not map on to 'the Iraqis should have no human rights'. But there's another way something can be about an event, it can allude to it. If we say 'The Mother of all Battles' we are alluding to Gulf War 1 - and we can push that allusion as far as we want, or stop there.
Ways that 28 weeks later alludes to the Iraq conflict include things like the response to a threat being massive, destructive and ineffective. About a war on horror which turns you into another horror. About 'containment' being exactly what forces your enemies to spread across the world. About a response which kills the innocent and colludes with the real source of the danger. About ordinary soldiers being ordered to do unspeakable things. And so on.