There is something we call the universe. We sometimes forget that we are always a part of this universe. We cannot jump out of the universe. We don't know exactly what the universe is, but nobody can deny that it is something and that we are a part of it. Most people usually forget that they are not separate from this universe. They forget that they are never separate from that which exists independently on our individual existence.
When we practice zazen and let go of body and mind, that means when we let go of "myself", this separate "self", then that mysterious thing called the universe, which is something we don't have a clear definition of, this so called universe grabs you and returns you to itself, like a mother reaching for a baby and putting it to her breasts. So when we let go of that individual self while practicing zazen, the universe acts like a mother who takes her baby back to her breasts and feeds it. So instead of "become one with the universe" we could say "go back to your mother, mother Universe". Of course, when a baby is held at her mother's breasts, the baby is still a baby. At the same time, it becomes one with her mother. So there is nothing special about shin jin datsu raku, letting go of body and mind, it is just like coming back to your mother, and becoming one with your mother. So whether you call it spiritual or not, it doesn't matter. What matters is that you really let go, when practicing zazen, let go of the notion "I am separate, I am individual, I am somebody else". When the baby gets milk, when it is nourished, the baby refreshes and is really happy and alive. That's what we can experience when we practice zazen - we return to our universal mother and get nourishment and then we become alive, happy and balanced.
To say "I want to become one with the universe", or "I want to have spiritual experience" or "I want to attain enlightenment", is all unnecessary. Instead we have to let go of that "I" and just sit. Just sit doesn't mean I just sit. it means just sit without I. Shikantaza does not mean "I just sit", it means "just sit". So shikantaza is not individual. When we just sit and let go of "I sit", we naturally let the universe grab us, feed us, look after us and nourish us.
Kodo Sawaki said "You don't practice zazen, zazen practices you." That means zazen takes care of you, it looks after you, zazen is not something that only you practice, in fact zazen is the essential principle of the mysterious universe. "Zazen practices you" means that the universe comes back to you and looks after you so you don't have to try hard, you don't have to do anything, you just sit in the correct posture and let go. Like that zazen practices you.
So instead of attaining enlightenment, enlightenment grabs you. It fills your heart, it fills your body and mind and you become you. You become you because you were willing to let go of the rigid, fixed self that is separate from the universe. So when we practice zazen and let go of the fixed self, everything returns to its original state.