Poem found here: "Long Finger Poem" by Jin Eun-Young
Is this an Ars Poetica poem? Most likely. But first let's talk about what's actually happening, "I'm working on my poems and working with / my fingers not my head." This is what the poem is all about. Or rather this is the thesis statement in the Ars Poetica. When writing poem, don't think to much and let the fingers do the writing. The poem then continues to explain how the fingers work for poetry.
"Because my fingers / are the farthest stretching things from me." Creating distance. "Look at the tree. / Like its longest branch / I touch the evening's quiet breathing." What this distance creates is sensation, experience, something tactile. If the work came to from the mind, would the speaker still experience the "evening's quiet breathing" or "Sounds of rain The crackling heat from other trees."
"The tree points everywhere. The branches can't / reach to their roots though." Even when reaching for the experience, the originator of the branches, the tree is rooted to the ground. But note that the branches are reaching for something it can't work with, can't experience -- being rooted and continuing to search with the branches leads to, "Growing longer they / grow weaker also. Can't make use of water." But roots can make use of water -- practical versus the aesthetic ins some ways.
And even though physical growth comes, "Rain falls." the metaphor continues to grow and gets "weaker," "But I'm working with these farthest stretching / things from me. Along my fingertips bare shoots / of days then years unfurl in the cold air."
Regardless of growing weak, the speaker continues to branch further to experience sensations or explore somewhere out there for the sake of poetry. Even if the branches have nothing, they can still unfurl and feel.