I do find that question intriguing. Those scrolls were more than poetry; the ancients did notice that willow bark soothed pain. In fact, the active ingredient is salicin, which our chemistry later refined into acetylsalicylic acid – the real aspirin. So, rather than a romantic metaphor, they were picking up on a genuine bioactive compound. Of course, their methods were crude, and their understanding of “compound” was vague, but the empirical observation was solid enough to survive millennia.