First off, they didn't have backups. When your characters die they are simply marked as dead, not deleted. This means that they can restore dead characters. The official rotmg forums and wiki were deleted permanently, which is why they had to scramble to put a new forum online. Also, Kabam has probably created forums for around 10 games now. They are experienced in setting it up and could get more than just the rotmg employees working on it.
Finally, I don't know why you're impressed by the new nexus. That would take a single person and hour or two max to make.