I don't know of the paper you referred to, but I have an analogy for you:
Religion is to spirituality (or spritual fitness) as GPA is to professional competence
While religion can be compared to college and the neccesary education to grasp a subject, there is little, if any, correlation with grades and professional competence or success.
We all know people who are religious, but we know are not spiritual. This can be evidenced by a complete lack of empathy, a complete and total contrdiction between their stated beliefs and action... I am sure many could add to the list.
I have heard many people say "I am spiritual, but not religious." I have no doubt this is true. However, like any human endeavor, spritituality requires a framework, a starting point, knowldege and some level of discipline.
To be completely spiritual without any connection to any religious beliefs denies you of both a starting point and a framework. Any knowledge you get will be random and spotty. Without a belief system there can be no level of discipline at all, for all your efforts towards spiritual growth will be without direction or goal.
It could be said that spirituality without any religious beliefs at all could be compared to a sailboat without a keel. You can trim the sails wherever you want and even move. However, without the keel, no matter where you set the sails, you will only go directly towards where the wind is blowing.