Inaccurate battery health indicator on iPhone 13 mini

You bought a used phone, which was discontinued in September, 2023. Backmarket is not an authorized Apple Seller, so there's no telling what you purchased or how it was serviced. They sell refurbished devices. To have a battery health of 100% it likely does have an authentic battery, but refurbisher's have figured out how to inflate the actual health of a battery. There's nothing anyone on this user to user only forum can do to help you with the phone you purchased. It has no warranty with Apple. If you're concerned about the phone, see if you can return it to the seller. That would be between you and them.

If you want an iPhone which is refurbished by Apple and comes with authentic parts and a one year warranty from Apple itself, buy from the Apple Refurbished Store --> Certified Refurbished Products - Apple

If you're unhappy with it, return it and go buy a phone from an authorized retailer instead.

"Cheap" is not always a good idea, especially when it comes to iPhones.

Also note the cycle count. 1470 is huge. In battery years, yours is already a fossil. Expected life is 1000 cycles.