You're absolutely right. From my non-lawyer understanding that's why the youtube-dl dmca mainly rests on youtube-dl showing illicit use within their code (download of copyright protected material) and not that the tool is theoretically capable. But it's indeed a slippery road.

There are other examples in the real world tho, where the distribution/creation of the tool is already illegal (e.g. certain weapons or explosives), because only reacting after damage is done is infeasible.