workflow-cps-global-lib not compatible with mercurial

I'm trying to use the workflow-cps-global-lib plugin with the mercurial SCM (legacy mode)

The dropdown to select the credentials for the mercurial checkout of the library doesn't contain the username/password credentials that I have configured globally.

I tried configuring the credentialsId manually in the org.jenkinsci.plugins.workflow.libs.GlobalLibraries.xml file and "reload from disk" but it seems it was ignored when trying to import the library I get Authentication required.