They might be referring to the fact that it may feature 4 player co-op, and requires you to actually take action, instead of just pressing a random assortment of buttons. I think they would have referenced Left 4 Dead, but lets face it. One game blatently saying "we are like this very similar other game about Zombies" doesn't bode well.
I think also the main meat of this assumption its like Borderlands may be as simple as " the levels are not liniar, you do not have a set goal on where you are going and are free to roam from point a to point z instead of going through points b-y. You are on a resort island, you can fight your way to get through wherever you need to. And being an island, like Just Cause 2, maybe vast! It could take you ages to explore all what is there. Finding places to survive. You could be in a building like structure in one portion, a bungaloo section the next, and a water park the next day, and you don't have to be told which to go to first.
Lets not jump the gun and say they are trying to hype their material as a knock off of Borderlands, lets try to assume what they may actually mean. Besides from what I've researched the game is meant to be melee based. So you won't be bogged by a series of guns. Like the initial picture shows, your best friend may be just a pipe.
Mowing down zombies from a distance like in L4D was fun, but imagine if your only way to defend against them is for you or for them to get right in your face. Melee is your escape option to get them off you, your first option is to run like your legs are made of fire.