It's used (or even built) by Mapbox? Very cool! Thanks for the pointer!
Leaving a couple of google results for interested folks like me:
* https://blog.mapbox.com/mapbox-%EF%B8%8F-valhalla-1fdda95ac4f
* https://blog.mapbox.com/valhalla-is-live-on-mapbox-d868e8d41611