It's just the ADFS SSO doing its SSO task. a well configured ADFS must not allow a user to log in with other credentials, that's a security issue. you can try running the browser as a different user (hold shift then right click on the browser icon) or you can use Firefox that has containers/container extensions. also test it in Application Guard window. it's more isolated than inPrivate window.
I do understand that ADFS is merely doing its job, but this behaviour was exhibited by not only Edge Insider, but by old Edge as well (in fact, I can still use old Edge InPrivate to "trick" ADFS and log in using my service account) and, since both browsers and ADFS are all MSFT products, I assumed this was "working as intended", that this is the way they were supposed to function.