To make things even more complicated, Apple now sells batteries to be installed by non-Apple people; even you. I don't know if these are reported as "genuine", since the battery is but the installer isn't. If that makes a difference to you. To me it looks like the battery itself is genuine. Have you checked Battery Health? If the battery was installed a year ago then you might expect a nominal 1-2% decline per month in battery health. iPhone battery and performance - Apple Support
Read this support article about iPhone battery and performance to learn about the battery health feature and what is considered normal performance --> iPhone battery and performance - Apple Support
To see the feature go to: Settings > Battery > Battery Health
"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions."
Legal - Repair Terms and Conditions - Apple
1.4
“A Do-It-Yourself (DIY) Parts Service. DIY Parts Service allows you to service your own product. If Apple determines that DIY Parts Service is available to you, Apple will ship to you a replacement part for your Apple product. The replacement part will be accompanied by instructions on installation and any requirements for the return of the replaced part."