Exchange.ManageAsApp is basically "impersonation", it does not give your app any specific permissions. This is the reason why you need to run step 5 of the article above, i.e. add the service principal to a given role.
Basically, you can skip step 5 and assign an Exchange-specific role to the service principal, which in turn gives you the flexibility to scope the assignment via management scopes or AUs. I have an example article about this here: https://www.michev.info/blog/post/4282/exo-rbac-improvements-1-limiting-application-access
Or you can refer to the official Microsoft blog post about this: https://techcommunity.microsoft.com/t5/exchange-team-blog/announcing-public-preview-of-role-based-access-control-for/ba-p/3688228
In your case, a direct role assignment for the View-Only Configuration role should do.