Announcements should be sent out according to the current registration status, instead of "to people not yet registered". This text should correspondingly be changed to "to people not currently registered". Consider the situation where the date/time of an event is changed. Those who registered may no longer be able to attend. The obvious solution would be to delete the previous registrations, in order to send a new announcement to the entire target group. However, those who once registered are excluded from announcements, even if their registration has been deleted, a behavior which is not clear from the "Note: this event's registrants are always excluded."
The intended use case is obviously to send a separate e-mail to those who already registered using "Email registrants". However, this does not include registrants whose registration has been deleted. Furthermore, why send two separate e-mails when one will do?
Suppose someone registers for an event and then has a conflict, requiring them to delete their registration. After a change of date/time, they might be able to attend.