Taking generic lipo/li-ion batteries and wiring them to the battery terminals doesn't count. Of course it's possible, but it requires some electronics know how. It's not just a drop in replacement.
I don't know why he didn't go with a lipo pouch. Having that thing sticking out of the back looks so unsightly, and it for sure won't fit in a pouch. A large, thin lipo pouch wouldn't increase the thickness much, so it might still fit in many pouches and also make it more comfortable to hold.