I'm pretty positive the Yogscast just spawned the villagers in. I had looked into this too and there were never villagers in the nether, even in old versions of Blast Off. It is not uncommon for the Yogscast to set things up off-camera for the sake of story. We can get villagers legitimately by using a healing potion on a zombie villager, but I think that makes vanilla villagers. I'm not sure why MCA is even included in the game unless they show up on other planets or something, but I think I've read other comment nobody has even found villagers on other planets either.