A poem should be motionless in time
As the moon climbs, (9-10)
If reality is defined by the temporal world, then a poem will have to leave all that behind and be "motionless in time." So a poem operates in a different kind of reality that's not restricted to all the typical physical stuff.
Leaving, as the moon behind the winter leaves,
Memory by memory the mind— (13-14)
A poem shouldn't be just about one individual's interpretation of reality based on his or her memories alone. It should "leave" that behind and rise above the scope of a limited interpretation of reality.
A poem should be equal to:
Not true. (17-18)
Well, if a poem should be equal to "not true," then we definitely understand the speaker's idea that a poem's version of reality shouldn't be "real" in any textbook sense. It shouldn't try so hard to pigeonhole life in any concrete sense but should rather be its own fluid representation of "reality."