That's true enough, but hardly a new thought that's going to surprise anyone.
I would say that an important point has been missed, though. As well as not feeling bad about killing them, a player is always going to sympathise with doing so. Whomever you are, zombies are going to be the "other", the "them" to an "us" which encompasses all of humnaity.
If you create a game about US soldiers killing soldiers of People's Islamic Terrorist State of Generica, everyone can get behind that, because everyone in the world is a US citizen, according to marketing experts. But if you create a game in which you play as the PITSG soldiers fighting the US, there will be arguing.
Zombies are more generic and easier for everyone to hate than even Nazis. They are a default badguy everyone is familiar with and nobody sympathise with.