A Norwegian newspaper got to see his letter of denial and it didn't list drug use at all. Rather they said he tried to enter the country to get a job illegally.
At this point, CBP's story keeps changing, and I don't trust anything they say.
https://www.vg.no/nyheter/i/OoKoRA/mads-mikkelsen-21-ble-nektet-innreise-til-usa