No. Giving something a meaning, even in relation to imagined, is not nihilistic. Psychotic maybe, yes, but not nihilistic.
It’s relatively nihilistic: a pagan, i.e., someone who sees “this” life as meaningful in itself (i.e., not merely in relation to something outside it) is the only kind of person that is not at all nihilistic with regard to “this” life. Compare the well-known joke in which an atheist tells a monotheist that the latter is almost as atheistic as the former: the former believes in only one god less…
What you are doing is stretching this definition too far.
You can:
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Give meaning to something on its own.
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Give meaning to something in relation to something else (in this case, to something imagined)
The end result is – you’re still giving it a meaning. Period. So it’s not nihilistic, relative or not. Just because you don’t assign a meaning in the manner that you prefer doesn’t make it less of a meaning, i.e. nihilistic. In the end, meaning is still a meaning.
Well, whatever. If you want to make an absolute distinction between your “fit” believers and “unfit” unbelievers, go ahead. I’ve suggested a spectrum of fitness display that ranges from atheism to panentheism, with monotheism being closest to atheism and thereby to the ultimate fitness display in that regard.
Unfortunately, to me it is impossible to imagine fitness without virtues.
But those virtues are means of upbringing of strength and power.
Religions are nihilistic inasmuch as they deny this “strength and power” and put an abstract nothing, nirvana or god as final goal.
If you can not imagine this god, then it is nihilistic.
In my opinion the sickest people can reproduce today while the healthiest can die off because they cant find to each other.
When Nietzsche said “when Christian morals perish, it seems that they were the only ones”. Because people have lost all others from their minds, the ancient morals were pushed aside by Christianity. What can lead us back to those ancient morals (and powers) is one part of the Christian morals which doesn’t belong to Christianity at all - truthfulness.
And reproduction depends on similar thinking - therefore values lead to reproduction:
A proper reproduction would be: “I respect the wisdom of the highest race to reach their power and to overcome their power later.”
Truth can never be told so as to be understood, and not be believ’d.
I didn’t really have much time to really think about it, but I will make a few comments on things that’ve been going through my mind so far.
To me, nIhilism is essentially a negation of meaning; or meaninglessness. In your given example, a religion would be nihilistic on the account of chasing after something meaningless and incomprehensible to them. By meaningless and incomprehensible I mean something that cannot be understood, even if a symbol is assigned to it that can be recognized. It’s like trying to get a meaning of a whole sentence written in foreign language by knowing the meaning of only one word in that sentence. (It means something, something, but I am not really sure what)
Alternatively, a person may argue (I think some philosophers did (Spinoza?)) that we, on the account of being part of this whole, can intuit it, to some degree. A vivid dream I recently had can perhaps be used to illustrate this. In my dream, a person that I feel close to was represented to me as a walking dead, and I mean complete with decaying flesh and swarming flies, the awareness of which, I did not disclose to that person. In the dream, the person was not aware of their state, but I felt heartbroken as if they were fading away from me and I was losing them, and felt that all I could do is to try my best to prevent this person from “disintegrating” even further than they already have, and I instinctively I acted very supportive and gingerly towards this person. The imagery of this dream hit me emotionally when I awoke, and I thought it was just horrible that I would imagine this person in such a state. (What’s wrong with me?!) Later, after consulting some dream dictionaries and reflecting back on it, the meaning eventually became clear to me. In waking life, I did sense that this person, who was struggling with personal issues in their life, suddenly withdrew themselves from me and became aloof, which I remember saddened me to some degree, so it makes sense in that the imagery represented my perception of their fragile state. (Upon even further reflection, I realized the dream was just as much about my own impotence at being able to assist them). Another thing I’ve learned is that I didn’t give it much thought/weight at all in conscious waking life, but apparently my subconscious was really affected by it. The meaning only became clear to me after consulting dream dictionaries. Now, I find it puzzling in how a dictionary may have an access to a meaning that I myself may not know, but which rings so very true, and the material of which I myself produce in my subconscious. Why was (my own) imagery meaningless to me at first? Can a meaning be formulated and understood by the illogical and irrational? Exactly how much role does the subconscious play in the formation of meaning? 
This topic is not about nihilism.
Nihilism occurs when morals detach from reality. Only reality, something real, can redeem morals.
When others make a trial to persecute someone for breaking moral laws then they are nihilists.
In other cases people would probably turn back on those who break customs as “bad” people.
Ignorance instead of punishment, that is a moral life with purpose.