I normally respect anyone with the word "Harvard" attached to their name, as is the case with psychologist Kurt Gray. However, his interpretation of this experiment sounds like a collection of bad causation-from-correlation assumptions.
Just because a subject, on average, holds a five pound weight for longer if they have donated the coin to charity does not imply that the moral viewpoint of the person caused them to be able to do so. There is no conceptual link between those two ideas.
What is more likely true is that a healthy, and therefore fit, and therefore stronger, person has less of a reason to worry about losing the coin (i.e. their survival instinct is not evoked), and therefore they will be less likely to take the coin for themselves. In other words, it's probably the strength causing the moral approach, not the other way around.
It would be very nice if it were the other way and the study made sense, though. It would really help to prove the words, "The arc of the moral universe is long, but it bends towards justice", by Martin Luther King Jr. After all, if people were always stronger when they were morally thoughtful, then moral thinking (or to be more specific, altruism) would overpower selfishness in the majority of overall decisions.
Concepts like morality and its connection to strength are not so easy to conclude on in the real world.