Some interesting comments cleared up things and confirmed my suspicions. The first one came from K7JOE, who lives in China.
“I live in China. I see these all over the place. The UV-82 Baofeng batteries that I buy are direct from Baofeng and are 1800 ma. Breaking the battery pack open (that’s what you do when they cost just $10 USD each) the cells are clearly 1800 mA Li Ion cells….). I fully suspect the 2800 mA batteries are just misprinted/mismarked. Interesting radio. I like the dual PTT switch on it — it’s handy when using multiple bands / dual receive. Great deal for the price.”
The second one comes from Martyn (radio-mart):
“I asked Baofeng and received conformation the battery is misprinted, they didn’t know until I pointed it out. The battery is 1800mAh.”