Each Office 365 tenant does not have a personal Exhange server. The sending smtp server is being shared and can be verified with the email headers. That is how this flaw works.

I feel like an idiot because I see the same behavior with the variable 'not in my organization' with transport rules and how many false positives I've had. I can clearly see the usage of different exchange servers being used in my environment when my company acquires a bunch of users. It falsely flags them because they are using a different shared exchange resource.