You're giving the quote a bizarre and implausible reading. Of course that Reagan (if he really said this) meant it to imply that a marine does make a difference, and a positive one. Any normal person interpreting the quote in good faith would make that assumption, whether or not they agree with the premise.

(Besides, googling for the source of this quote, I can't find any reference to a concrete time and place where it was uttered, nor to the rest of the speech or dialog whose part it was. This strongly suggests that it might be apocryphal, though of course I can't conclude this with certainty.)