Is the poetic verse: "Were it not for You in its planting, the bough would not blossom," – whose pronoun refers to Allah Almighty – considered to be from the doctrine of Ḥulūl (divine indwelling) among the Jahmiyyah?

This poetic statement has no relation to the disbelieving creed known as Hulul (incarnation/indwelling) and Ittihad (unification). Rather, it is a beautiful statement with no fault in it. Its meaning is that were it not for Allah Almighty, the seedling would not have grown nor would the branch have sprouted. He is, in truth, the Causer of causes and the Grower of plants, as in the Almighty’s saying: ﴿Is not He Who created the heavens and the earth, and sends down for you water from the sky, with which We cause to grow gardens full of beauty? It is not in your power to cause their trees to grow. Is there any god with Allah? Nay, but they are a people who deviate from the truth.﴾ (An-Naml 27:60). One must think well of a Muslim and interpret their words in the best possible light, especially if the words can only bear a sound meaning. The creed of Hulul does not distinguish between the Creator and the created, nor between the worshipper and the worshipped.