Hey! Hey listen! (and watch!)

It is a Feature Trailer. Here.

https://www.youtube.com/watch?v=2c8EEXL3L8c&list=UUGIY_O-8vW4rfX98KlMkvRg

Do the mass of generic ennemies in DW do any damage or are they there just for decoration?

They can, yes. What they're showing you is on the easiest mode, where the enemies just sit there being braindead. On Normal they'll actively surround you in an attempt to get in your way, although they're still nuisances in comparison to generals/officers/bosses, which are generally the real threat. Most of the difficulty comes from having to manage the map in addition to the hundreds of enemies onscreen, which is definitely not an easy task.

On Hard/Chaos though... let's just say things get a lot hairier.

I loved the part at the end with the chickens, makes it feel a little more legit.