Yeah, the reasons you mention are probably why they are even able to do this, but it's still pretty extreme that the DNA repair mechanisms do not seem to get any weaker at all.
Perhaps, they are so protected against cancer due to an evolutionary 'accident'?
This is an interesting thought, and I cannot really make much more than a guess on this point. But it's very interesting that they seem to be naturally resistant to cancers in general, because most cancers are very different from each other in terms of how they attack the body, so this could maybe have potential for future treatments.
It's very cool to see that you like these animals so much that you even wrote a first and only biology post about them. But I get why you like them; it's difficult to find an animal that is as unique as this one.