If I read it right, it resists radiation damage to itself. That boost is to the lifetime of the shielding, not it's effectiveness at protecting the contents?
I think it does the second indirectly, by staying around where it is supposed to be for longer.
If you lose more of your radiation shielding on a trip to Mars, you need more shielding to start with to get the same average radiation protection
Aluminum just gets more brittle when exposed to high energy particles. It's not like it goes anywhere. It will still perform about as well as radiation shielding although there are better materials for that. So if you want to use the aluminum for structural purposes, especially with gravity present, you'd have to make more robust designs just to account for any embrittlement which also means more weight and therefore more fuel to get the parts off earth.
being able to keep the metal strenth is very important, this kind of issue makes half french nuclear reactors down for some months…