How do you tell Apple Intelligence an email it thinks is a priority is not important

You don't. Priority Email simply reviews the email to determine if the contents require immediate action. It is not for email filtering where you can choose which ones are a priority. You may even see spam showing as priority if it recommends you need to take some action today. For spam, you still have the option to mark it as junk, where it will then filter those emails. If the contents are a Boarding Pass or Calendar Invite, it will always show those as a priority. You can either choose to have Priority Email turned on or off.