Each subscale score is doubled, because the DASS-21 is half the length of the full DASS. The severity bands apply to that doubled score, so it can be compared with DASS normative data and other published DASS results.
“Mild” does not mean a mild disorder. The labels describe where someone sits in the general population — “mild” means above the population mean but still well below the typical severity of people seeking help. The scale's authors call the labels arbitrary and warn against reifying them.
The DASS is not a diagnostic instrument and cannot diagnose depression, anxiety or stress. It indicates whether these are having a significant effect at present; a high score needs exploring in conversation and may warrant referral for a clinical interview.
The bands differ sharply between subscales — a doubled score of 20 is “moderate” for depression and stress, but “extremely severe” for anxiety. Read each subscale against its own band, never against the others.