I just had to tweak the glide.authenticate.sso.redirect.idp property, here you put the SysID of the idp - in my case it had a different value from the migrated one.

I also ensured that the 'migrated idp' was set to active and was my default and I disabled all other ones (why have something active when you don't know what it does)...

What I found is that people who tried connecting before I configured SSO were not automatically authenticated and were instead dumped on the logout screen. Once I had them clear their cookies the login behavior was as expected. Anyone who tried connecting after I configured it worked fine. I found that the best way to test this is to run an inPrivate / Private / Incognito window.