Quantum immortality...is about whether you can live forever by defining yourself as a person who doesn't die....Experiment: Attempt to define yourself as a rock, thus increasing your lifespan. Did it work?

The assumption of quantum immortality is that once some branches of "you" are gone, then you have to define "you" as the remaining branches. It's about the impossibility of expanding the definition of "you", not the possibility of expanding it.