battery replacement

I sent my IPhone 6S to Apple for battery replacement. Phone and battery are 6 six years old. Apple sends me an e-mail...

Your product arrived at our repair center, but our technicians weren't able to process your repair request. We’re sending the product to you along with a letter that provides more information.

i called Apple support and ask why the battery was not replaced. Tech said battery passed some sort of diagnostic test. I prepaid for a battery replacement, after heavy use and 5 years of service, I don’t care what the diagnostic said, the battery is toast. Battery health says the battery is degraded and needs service. What’s the story Apple