It's kind of hard to tell by the map, so maybe this drawing will help.

Bors is standing at the top of the landing, five feet ahead of him, Maloria is standing on the stairs, probably on the top one or two steps. One zombie is standing a few feet below her, while another is currently behind that one lurking somewhere in the dark.
So basically, as it stands right now, the only way the Zombies are gonna get upstairs if for them to get past Maloria, and then shove past Bors who's standing directly behind Mal.