They would be able to move though the space; they simply cannot enter into a space within that 30 feet from outside that range. Otherwise, by that logic any undead within 5 feet would simply be unable to move, as moving away from you requires entering a space 10 feet away from you, which is a space within 30 feet of you. Turn undead is not designed to be used as a means to root undead in place, it's designed to make undead flee away from the person turning them.
A turned creature must spend its turns trying to move as far away from you as it can, and it can't willingly move to a space within 30 feet of you.
It could also be ruled that, because it cannot flee, being forced to move away from you requires it to unwillingly move closer.
Previous editions stated that undead would move away from you as fast as they could, and if you trapped them where they could not flee without moving towards you they would stay still, but moving closer to such trapped undead would break the the turning effect on them.
One option is that the undead between you and the door that are turned flee through the door by moving normally or using Dash action, while those that are trapped between you and the wall (who cannot reach the door without moving towards you) would move as far away as they can within the room, or stay where they are and may use the Dodge action (per the description of Turn Undead).
Another ruling a DM might make is that if a turned undead is able to end its move further away from the turner than it currently is, then it may move towards the turner in order to get away.
Ultimately the decision at this stage is up to the DM until further rules regarding turn undead are released, or one of the designers chips in.