I don't think anyone dismiss the data taken only from the military because of their workforce size & variety of positions (as indicated later) but because their task/goal is very unique to other private companies where the (primary) goal is making a profit, and the fact that at the military, sizable proportion of the employee might be in a high stress, life-or-death situation (the data was taken under Iraq/Afgan war) .
All in all this seems like a "checked all the boxes for a good study; but still whimsical result" kind of study. (I guess that's why it went to a low impact factor Journal)
I still would prefer HR people take recommendations from this article, such as: "Measure happiness in both employees & job candidates" at face value rather than wasting their time in BS Personality-type and pseudo-IQ tests for hiring/managing project groups
Edit: Might also add that (to me) it makes no sense to equate "getting a medal" to performance, when they also claim "Military also have non-combatant jobs!" as the basis for generalization of the study; yea but your success metric doesn't take that in to account? Does military-chefs get medals for extra-ordinary dishes? Does safety technicians get medals for "extra-ordinary safety"?