If the manual says max 8GB, then that means it is locked in the motherboard BIOS to only recognize that much. It's possible that those some people you are referring to may have found a hacked/modified BIOS from another Dell PC model's motherboard which took a max of 16GB.
Otherwise there's no other way around it other than to replace the motherboard with an aftermarket one from the likes of ASUS, MSI, Gigabyte, ASRock, etc. FYI I would not recommend experimenting with modified BIOS's...they can brick your motherboard forcing you to buy another one.