There's two things to consider
The Mendix Portal now offers the option to bring your own IdP. This is what your screenshot refers to. This will depend on the company you belong to.
Mendix Apps can be extended with different SSO mechanisms, including the MendixSSO module, which allows to delegate authentication in your App to the Mendix Portal.
Combining both allows to login to an App with MendixSSO through your company IdP registered in the Mendix Portal.
SSO is enabled for the accounts associated with your company.
It's not bound to specific applications, so if you invite externals to a project they will still use their own login mechanims.
I finally have this up and running.
the confusion is that there are multiple different SSO documentation
1. allowing users to log into your company sprintr using SSO (not what we want).
2. using open ID to log in via sso (Also not what we want)
3. using SAML to log into via SSO
this is way easier then I expected.
install SAML.
add the startup MF.
create a user for the SSO admin of the client company so they can configure this.
(they need to match some data)
this also needs to be done on a live location (Not local host)
and the person configuring and testing this will need an account in the client IDP, and also be connected to a 'group'