The audience at the time of writing weren't herbalists either.
The audience at the time didn't have CVS - they used herbal remedies on a regular basis. I would expect them to have been aware in general. Some folks have the idea that people of earlier periods, being ignorant of today's known science, were just... generally ignorant of everything in their worlds, which seems unlikely.
Shakespeare liked wordplay and puns, so I expect that played into the scene.
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