Assuming this module is made in Mendix 6, then what you really want to do is get the latest version of Mendix 6, which is 6.10.19 and can be found here. Mendix 6 was released at the end of 2015, so it may be possible this module was created in Mendix 5. Mendix only supports the last 2 major releases, so you wouldn’t want to download an older version. Instead, Mendix will allow you to open an older module in a newer modeler so I’d recommend starting in Mendix 6. Beware, though, that there were some big Java changes between Mendix 5 and 6 as outlined here so the process may reveal some errors. This module hasn’t been updated in many years, unfortunately, so there isn’t any guarantee that it will still work. You should be able to open it up, however, any maybe take some logic and re-create it in a new app.
The DigiD app page does not have a download button on the page anymore, so I think there is no possibility to download at all. If you are still interested, you could check with the author of the module, Bart Poelmans (https://developer.mendixcloud.com/link/profile/overview/9993 using the `Contact me` button).