Refurbished iOS devices from Apple generally come with new batteries. They replace the batteries in Macbooks if the existing battery is already defective. That doesn't mean it can't have 3 years of use on it already, and then fails 3 years sooner, it just means it's not already below the threshold for immediate replacement when you buy it.
Apparently many of them are replaced. I found a thread on Reddit where a few people were saying they've bought refurbs and they often have 0 or 1 charge cycles on the batteries, indicating they were replacements.
