Let's step back even further and consider Unity's pre-DOTS "entities" and "components." These do not take the data oriented approach, are not typically even classified as ECS (e.g. because they lack the System aspect of that design). However, the components are clearly chunks of concrete data (transforms, meshes, rendering parameters, rigid bodies, etc.) rather than interfaces/protocols.
This is the sense in which ECS means "component." That book is not relevant to this sense.
