Am I missing something?

Craft is from Invision, is a plugin for Sketch. Prototype is one of the feature in Craft thats making prototypes within Sketch app.

Once you makes some prototype actions, you will need to use Craft Sync to sync your design and the interactions to your InVision projects. This will not work with other tools like Zeplin.

You can read more about Craft by InVision here

Zeplin is a tool that handoff to developers so that they can know the measurements, fonts, colors etc used in your design. It’s not for prototyping. But InVision have launched a similar feature which is Inspect. Since Inspect is launch, we have stop using Zeplin anymore. You can read more about InVision Inspect here: