At one point-- after hearing the line about the "machine that makes machines"-- I thought that maybe the zombies were actually 'badly-formed-robot-antibodies' created by the damaged,
'snarling' TARDIS to protect itself against the intruders. Turned out not to be the case, I realize.
Still, not sure why dying, irradiated future versions of the salvage guys & TARDIS crew would be stalking their present-day selves.... (?)