The phrase "healthy lifespan" refers to the number of years a person can live while staying in good health, without any major illnesses or disabilities. It focuses on both the length of life and the quality of life. Full definition
Eliminating senescent cells might one day extend healthy lifespan in people, but these drugs will require careful study to discover any undesirable side effects. (nih.gov)
In addition to disputing the relevance of your source trial (which doesn't appear to offer a duration or suggestion that the tests increase healthy lifespan of the mice in question), I'd like to point out that protection is less beneficial in the long term, because you can not stop damage from occurring completely no matter how well you defend an organelle, and that damage will accumulate over time. (lifespan.io)
Based on mouse studies conducted both at UW and elsewhere, they anticipate the drug «could increase healthy lifespan of middle - aged dogs by 2 - 5 years or more.» (mnn.com)