Yes, we know all of the paradoxes in "Ars Poetica" can make a guy feel like a hamster on a wheel. But we promise there's some "truth" to it all. And that truth rests not in math equations of "equaling" something, but rather it rests on forgetting about all that "meaning" stuff, and instead focusing on "being."
Lines 17-18: Even the speaker's tone here points to the difference between "equaling" something and being "not true." One sounds like a math equation while the other gives the impression of simply existing in a more truthful way. A poem shouldn't sound like a math equation but should rather be allowed to be free and "truthful" in its own way.
Lines 23-24: Here's our big takeaway, which is of course "not true." A poem shouldn't mean but be. It shouldn't be concerned with truths and meanings but should rise above all that and make us feel motionless like that moon climbing in the sky.