Once again, you have been had by a poorly trained dealer making up features as he goes. What else did he tell you that is strange or crazy?
The battery for the fob is a CR2032, easily available at any discounter or mass merchandiser. They are a pretty large button battery.
So, that means they don't recharge in the slot. The manual also indicates that batteries in the fobs are replaceable and if you have SKS you will probably replace them about once every 2 years, or when you feel that range or response time from the SKS system is limited.
Anyway, a recharging system in the key slot wouldn't be all that useful if you had SKS as the key would be in your pocket, not in the slot.
Do you have SKS on your Prius?
Do yourself a huge favor and read the manual. Best investment in time I made when I got the car. And keep reading the forums and asking questions.