It has been confirmed by DataMapper’s core developer Martin Gamsjaeger (snusnu) that it is a bug.

In short, created_[at|on] can be manually overridden, but updated_[at|on] cannot.

The workaround is simple, do it in two steps, for example:

job = Job.create(
  title: "Web Developer",
  created_at: Time.now - 2
)
job.update!(updated_at: Time.now - 1)

Please check the bug ticket to see when it’s getting fixed.