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:
This package contains type definitions for ol (https://github.com/openlayers/openlayers).
Version 6.4.2