Yes, cytotoxicity is the ability to destroy cells. Most CD4 T cells are not cytotoxic, regardless of the host's age. In the Supercentenarians studied here, it was observed that the majority of their CD4 T cells were cytotoxic, which is a very surprising result.

Having more cytotoxic CD4 T's could give the immune system better ability to destroy invaders or cancer cells.

Even more surprising is that a large percentage of these mutant CD4 T's are clonal, all traceable back to one individual (T cell) that mutated.

It's been shown that supercentenarians enjoy resistance to disease over their entire lives. This research shows that may be due to their unique immune systems.

That could be one potential consequence but this publication just lays out the observation and clearly states this at the end - "It should be noted, however, that antigens recognized by clonally expanded TCRs are not known, and further work is required to characterize CD4 CTLs in supercentenarians." IOW, the actual characteristics of the T-cells in this sampled population is not known. As frustrating as it sounds, more experiments are needed to say with certainty what is correlated, what is causal etc.