But devise is only single tenant and doesn’t offer a web admin view for managing the integration?

Devise does not impose anything on the number of tenants. I run a Rails app with multi-tenancy and SSO support with Devise. As someone has mentioned, although it does a lot of the work and it keeps well organized, you still need to do the UI for configuring and some backend logic.

Devise doesn't support SSO, but it supports omniauth, which supports SSO, right?

You certainly can write and maintain code to add multi-tenant sso, but since omniauth does not support any configuration storage, you have to add that yourself.

Devise does integrate with SSO, but you still have to set everything up yourself for all the main providers which takes a good bit of time with a lot of custom frontend work needed and a bit of extra backend as well. It would be nice to have SSO out of the box without the need for a bunch of manual setup which was kind of the promise of this bullet train thing.