According to https://github.com/dcos/dcos-cli/blob/master/design/login.md we support 4 login client methods. However looking at the old Python CLI there also seems to be browser-prompt-oidcidtoken-get-authtoken which exists.
I think it was mistaken with browser-prompt-authtoken, which requires the user to continue the flow in the browser and then paste the ACS token to the terminal, instead of the login token.
So while login to DC/OS Open works, it misuses the browser-prompt-authtoken in the mocked provider list (as the /acs/api/v1/auth/providers endpoint doesn't exist on DC/OS Open).
This problem was discovered while working on https://jira.mesosphere.com/browse/DCOS-41848, this is a test relying on a properly configured browser-prompt-authtoken login provider, the new CLI treats it as a prompt-oidcidtoken-get-authtoken provider.