Using Outlook for iOS for personal email and calendar management
Yes, the wording is confusing, and I want to clarify one point from my previous reply.
Microsoft says you must “sign in to your Outlook.com mailbox at least once in a one-year period,” but it does not clearly state whether simply receiving mail in the Outlook app always satisfies that requirement. See: https://support.microsoft.com/en-us/outlook/email-is-missing-from-your-outlook-com-inbox
It does not specifically say that you must use the website. However, if you want to be 100% safe, I would recommend signing in at https://outlook.live.com/ at least once a year. It only takes a moment and removes any uncertainty.
Since you already use the Outlook app regularly, there is no reason to be concerned. The occasional website login you mentioned is a good precaution.