The article suggests that decline in nitric oxide availability as you age under pins the effect studied. Citrulline has been found to be an effective route for improving nitric oxide levels by synthesizing into arginine, which is destroyed in the gut while citrulline is not and readily converts into arginine.

https://bpspubs.onlinelibrary.wiley.com/doi/full/10.1111/j.1...

… in humans