Seams like that is a statement that can not be made solely based on the stated connection. To me the initial thought that occurred when reading was something like:
"yeah well smoking is dangerous. People with more cognitive power are presumably better at reasoning themselves out of smoking; Given the fact that smoking is known to increase the risk of getting cancer."
If that description describes something that is a big factor in determining a persons smoking habit, smoking would not necessarily cause a cognitive decline as implied in the recommendation.