Conditional Access policy is not set directly on a client (public/native) application, but is applied when a client calls a service, quoting the document here: Conditional Access: Cloud apps or actions.
The policy blocks user access whenever the client/app is calling SharePoint or OneDrive, while Outlook mobile app uses OneDrive for storage purpose. That is why Outlook mobile app is not excluded from this policy and asks for enrollment on non-compliant devices.
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