Seems reasonable, but maybe not exactly a weapon. It is used to subdue someone, but not as a threat. Should be a parallel class. And while the intended assault is a sexual assault, a date rape drug still incapacitates you, which easily classifies as kidnapping, unlawful detainment, etc.
A stick of dynamite probably counts as a weapon too (I would think?), in a legal sense. I'd say it's a reasonable perspective for the legalese. Also, cars have been considered weapons in some cases.
This can of course lead to problems where the police can turn everything into a weapon even if it was never intended to be one, but I'd argue this is less of a problem with these rape drugs, as this shouldn't be a thing people normally carry with themselves for legit reasons.