Data source for bus isochrones

How are isochrones calculated for buses? Are the actual timetables used as the basis for the data? Or is the distance travelled by a car adjusted to the average speed of a bus, thereby reducing the area covered by the isochrones?

The latter scenario, based on the average road-type specific speeds defined in heavyvehicle.json.

Thanks for the reply.
So no GTFS data is used to generate the isochrones?