In the poem, people and nature literally become one as the faces in the subway become flowers on a tree. The analogy between faces and flowers is not just a simile, which would say that one thing is "like" another. Rather, it is metaphor: the poem implies that the faces are petals on a tree.
The poem isn’t trying to make the subway seem like a natural setting. It’s trying to create a fictional space where both natural and unnatural settings can co-exist.