The thing is, the exclusion zone isn't uniformly radioactive. The hottest spots are not areas that wild life would normally spend a lot of time near.
Then there's the fact that the way we're all taught about radiation and cancer is just flat out wrong. The Linear No Threshold model that most people know was actually created by the Rockefeller Foundation in an attempt to slow the adoption of nuclear power.
Combine those two factors, and you get stories like this, where researchers are shocked that higher than average radiation exposure doesn't equate to a simple linear increase in cancer rate.
Not that these wolves haven't developed an increased resistance to radiation. But it's not a new thing. Every living creature on this planet has mechanisms to repair DNA from radiation exposure. These wolves are simply better at it now than generations past.