There's a mismatch in documentation between Account SCIM 2.1 Documentation and the link you've provided:

Anyway, you can obtain members for each group using /api/2.1/accounts/{account_id}/scim/v2/Groups/{id}.
Just iterate over group ids that you can obtain via this endpoint /api/2.0/preview/scim/v2/Groups and pass group_id as a parameter.