The Holiday impact insight email contains information about the holidays of other team members by default. If there are holidays close to each other, even if not booked on the same dates, these will show in the email sent to the manager or approver.
The Impact Insight will only show the holidays for the team that the approver is managing.
The team will be:
The employees who report to that user.
The employees who report to the user's manager.
The user's manager.