The use of "upright" in The Heart of Zazen reflects posture, but also relationships and appropriate conduct; see for example Reb Anderson's book "Being Upright" on the 16 Boddhisatva Precepts. For appropriate posture, a straight spine is most important in zazen, which ultimately, according to Dogen. has nothing to do with whether one is sitting, standing, or lying down. The classical lotus posture for zazen however makes being upright much easier to practice.
From the opening paragraph of another of Reb's books, "The Third Turning":
"A buddha is someone who sees the way things really are. When we see the way things really are, we see that were all in this together, that we are all interdependent. A great surpassing love arises from that wisdom, and that love leads a buddha to wish that all beings would open to this wisdom and be free of the misery that arises from ignoring the way things are. Buddhas appear in the world because they want us to have a buddha's wisdom, so that we will love every single being completely and protect every single being without exception and without limit -- just as all the buddhas do.""Cultivating Compassion" By Tulku Yingrik Drubpa
"The essence of all the teachings taught by the Lord Buddha concern taming one's mind. The point being to return the mind to its natural state of freedom, which is endowed with limitless qualities. The realization of this natural state gives rise to spontaneous compassion that is limitless and impartial ... give us the freedom to embrace the world by enhancing our capacity for compassion, love, and perseverance."
Compassion and love are part of (human) nature, but our initial social development integrates many other instincts. When passions and competitive proclivities are tempered through practice of meditation, compassion and love re-surfaces within our original and final nature.
TNH's poem is equally true if you replace "Peace" with Ideals such as Love, Happiness, Truth, or Compassion.