The @types/openlayers package (https://www.npmjs.com/package/@types/openlayers) only supplies type definitions for OpenLayers 4.6. This is stated explicitly at the top comment in file index.d.ts.
Are types for OpenLayers 6 available and, failing that, what should be the course of action for developing Typescript applications with OpenLayers 6 ?
Take a look at
@types/ol: