Martin Poirier Théorêt
No sorry never found a solution and I am not working for the company that needed that anymore.
They have a premium support contact and they were overwhelmed with issue at the time and they end up postponing the date of the mandatory requirement to implement Apple Sign In. I am still in contact with a the UX designer of the company (who’s a friend) and he never told me that they’ve released it. So I don’t know the current state of their (Apple) “api”.