This is just OpenStreetMap with another frontend on top, you can see this by going to the area[1] and going to the right, selecting layers and looking at the CyclOSM one.
Also, a bit lame that on mapy.cz, you have to click on the "and others" to actually see the OpenStreetMap credit.
Surely there's nothing wrong with using OSM as one of your primary map data sources? As far as I know, Mapy.cz uses several sources in the Czechoslovak territory, although abroad, perhaps quite a bit fewer of them are being used.
Also, a bit lame that on mapy.cz, you have to click on the "and others" to actually see the OpenStreetMap credit.
[1]: https://www.openstreetmap.org/#map=17/48.75343/19.87948