In jalā al-suyūlu ʿani-ṭ-ṭulūl, jalā is the perfect verb, al-suyūl its subject and ʿan al-ṭulūl a prepositional complement. Kaʾannahā introduces the comparison, referring to the uncovered traces.
In tujiddu mutūnahā aqlāmuhā, “pens” is the subject and “their surfaces/texts” the object; this reverses the more familiar prose order. Presenting both the writing and the pens that renew it gives motion to the simile.