That only works if you can put the app on your internal network; Notion is neither self-hostable nor available 'on-premise' w/peering, that's what people are complaining about here.
You can't magically put 2FA in front of some third-party public site. (You could from within your own network.. but that's perhaps worse than pointless!)
Re:Notion API – public beta
by ArchOversight on Thursday May 13, 2021 @07:33PM
(#27148822)
How does this solution help if the SaaS offering doesn't support alternate authentication providers and doesn't offer 2FA?
For example, https://www.authelia.com/ is open-source.