Attaching OneDrive File to Email in Outlook App
This modal is unrelated to iOS Files app — it’s a custom presentation by the Outlook app. The app displays common account providers in the list to help promote the ability to log in to these providers for the purpose of attaching files, with the ‘x’ button to dismiss this.

The ‘normal’ way of adding a storage provider to Outlook with or without actually having a file to add is through Outlook’s settings. Open Outlook’s settings using the gear icon from within the app, then choose Add Account followed by Add Storage Account. You can then choose a provider from the full list of available providers.