It might be a big deal. They used Yamanaka factors (chemical signals used to allow children to be young, rather than a thirty year old mother giving birth to a thirty year old child) to deage cells. That’s not new; deaging cells has been around for a few years. What is new is that they managed to deage some cells in a tissue, and then have them relearn to do their job from the untreated cells around them.
The thing about Yamanaka deaging is that if it’s done far enough, the cells in question become stem cells, and lose their specialization-they don’t know whether to be a brain or a nerve or what. Thus, there’s been a tradeoff between aggressive deaging (which can kill if you push it too far; an animal can’t usually survive as a blob of stem cells; also if the stem cells get confused they can cause cancers called teratomas) and insufficient deaging (a much more gentle Yamanaka regime has been shown to safely extend mouse lives, but it’s not powerful enough to stop age from eventually killing them). The breakthrough here is that a highly aggressive Yamanaka regime, powerful enough to heal thirty years of age, and powerful enough to cause stem cell reversion, can be used without leaving the stem cells confused and unintegrated: they become cells like those around them once more, and rejuvenated.
The optimistic view is that we could use this sort of thing to gradually refresh the entire body, only converting some cells at a time to ensure the others can guide them back to proper specialization. That could be true immortality.
The pessimistic view is that we won’t be able to deliver it reliably throughout the body, won’t be able to target which cells to revert so some won’t receive the treatment or too many are converted at once, that it might cause cancer eventually even though it seemed safe in this trial, that sometimes the respecialization process will fail, that you can’t turn parts of your brain to stem cells without losing important information and potentially dying if you do this often enough to defeat aging, or that other forms of aging damage will still kill you even if this works as advertised.
Realistically, in my opinion at least, this is probably either a way to reset the epigenetic clock in most tissues or very close to it. That’s fantastic news, but if we want true immortality from it, we probably still need to treat transposon propagation (genes that copy themselves in your genome, basically the genetic equivalent of cancer), possibly need to address system-level aging dynamics (are there aging markers between cells that wouldn’t be cured solely by healing the individual cells?), and need to see if this can be used on the brain safely.