How to fix incorrect warranty date on recently purchased iMac M1 2021?

Authorized retailers may have still had them in inventory for a while. My guess it an authorized retailer sold off their inventory of 2021 iMacs at fire sale prices last year and someone bought them up and re-sold them.

Exactly where did you buy it?

This would tend to indicate that you purchased a used or gray market machine from an unauthorized retailer.

As noted something is clearly wrong. The iMac (24-inch, M1, 2021) was discontinued Oct 2023, so at best the warranty would have expired no later than Oct 2024.