You don't need to, that's the thing. The site requests "are you over 18" and you use your ID to prove it without them getting any other information from it. Requires a phone with NFC, but the app is open source
If the app is open source, what stops someone from modifying it to always claim the user is over 18 without an ID?
And using someone else's Id and password is the same as every method of auth
It would be possible for them to provide an open-source app, but design the cryptography in such a way that you couldn't deploy it anyway. That would make it rather pointless.
I too hope they design that into the system, which the danish authorities unfortunately don't have a good track record of doing.