I would have thought poisoning someone, even with a sedative, would already be a serious crime. This makes me wonder if this is addressing a weakness in the force of law or if it’s political pandering to look tough on a class of crime without changing anything.

It is already. This offense is ususlly dropped since other crimes overshadow it and mostly the biggest crime matters in the german criminal court.

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.

Exactly, or a piece of wood. If you threaten someone with a 2x4 you may have intended to use it as a weapon even if it was technically just a piece of wood.

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.

In a fundamental level giving someone a drug like this without their knowledge and against their will is an attack on their bodily autonomy.

And bodily autonomy is a human right.