Sounds wrong because the extra syllable blemishes the aesthetic quality of the symmetry of the rest of the poem.
Planets do twinkle, in fact. Its just that we do not notice. And it is the hallmark of a truly good (and bored) scientist to correct a misconception, even if it is only superhuman vision that can salvage the fallacy.
The atmosphere is a sea of air. Air is a fluid. Which means it is not vacuum. Therefore it has a refractive index. The density of air changes with altitude, so the refractive index also changes. Any light that enters the atmosphere (except from the normal vector, obviously) is bent.
Stars are very far away. They are so far away that they appear as point sources of light. So if we were to draw a ray diagram for a star, we would only need a single line to denote star light. It so happens that due to the constant ‘flow’ of the atmosphere, some times that star light is refracted so that it momentarily does not reach our eyes. That is called twinkling.
Planets are not that far away. Since they are nearer they can be resolved as light sources with a dimension. So a ray diagram for a planet will have multiple lines denoting light coming from different points on the planet. That light is also bent the same way as star light. And individual rays also sometimes bend enough so that they do not reach our eyes. But there are enough rays that do reach our eyes that we do not notice the slight change in the planet’s brightness. Hence we do not notice the planet twinkling.