There is quite a significant amount of air in wood. It's very porous, pretty much a sponge of lignin with air between the fibers. Waterlogging it replaces the air with water increasing the density enough for it to sink. What do you think waterlogging does?
Yep, I'm not sure what baking soda is supposed to do. Could work to decrease the water's surface tension a little making it more easily penetrate into the wood, but I don't know that it would be significant.
I've always thought the ideal way to waterlog wood would be with a vaccuum chamber. Pump as much air out as you can, maintain a vacuum for a while. This might take a while as all the air makes its way out of the wood, then open up another valve and dump water in. As the pressure increases it should force the water into every pore of the wood.
Never tried it though because I don't have a vacuum pump.