Alert User of Phishing option.
The alert user of the phishing button doesn’t send email alerts to the user. What the button does is that it modifies the subject of the email to include an alert tag.
This is used when you don’t want to quarantine the email but want to “alert” the user that the email may be phishing.
If the email is quarantined then "Alert user of phishing" action will do nothing. The end-user (depending on your phishing workflow) should have been notified that the email is quarantined.