Of course in the real world this can't be proven, which is exactly why a one year sentence for murder is also purely a hypothetical. The point is that if we had perfect knowledge into the mind of an individual that could prove they have been "rehabilitated", why on earth would you keep them locked up? There's just no rational reason for it.

In practice judging rehabilitation with perfect knowledge is impossible, and so are one year sentences for murder. In the real world we would balance out the likelihood of actual rehabilitation with concerns regarding public safety.