About the most one can say about this is that manipulation of NAD+ levels is a better exercise mimetic than manipulation of sirtuins proved to be. What it helps to do is explain some of the underpinnings of studies that show a great deal of muscle loss is secondary aging - i.e. disuse. It also suggests that loss of capillaries is a more important mechanism in sarcopenia than other groups believe to be the case, though I think it very possible that this is secondary to stem cell activity loss. The evidence for stem cell decline to be the primary cause of sarcopenia is very good. [1] There is a faction that thinks capillary loss is important in all tissues. [2]
Go look at the studies showing resistance training to be pretty effective in seniors before getting excited about NAD+ boosters. The resistance training is probably cheaper and better. One still has studies such as the one showing that dietary nicotinamide doesn't do a great deal in mice [3] - like sirtuin manipulations, it helps fat mice a little, but is distinctly unimpressive otherwise.
Remember that calorie restriction extends life by 40% in mice, but probably five years or less in humans - the effects of all these stress response related mechanisms of metabolic manipulation work through a few overlapping core processes, and scale down with species longevity. That's why I see them as a dead end in comparison to other strategies.
[1]: https://www.urmc.rochester.edu/news/story/4788/stem-cells-ma...