“Imitation” very strongly implies that it is not in actuality the thing being imitated. Imitation butter is not butter. Imitation crab is not crab.

These medicines are the same chemical, therefore the same product.

I will however grant that while calling store brand painkillers “imitation ibuprofen” is nonsense, calling them “imitation advil” is okay because advil is a brand. Though in my opinion it should be avoided because it carries an implication of inferiority that is simply not the case.