Yes, rotting plant matter returns all the carbon to the air - by definition. That is part of the cycle of life, and proves the laws of conservation.
The point of making biochar (which is fully activated carbon, made at higher temperatures than charcoal) is that you do NOT burn all of the carbon, and put what carbon you can in the soil.
When you do pyrolysis, you gasify all the volatile material out of the wood, and that releases more of the energy than low temperature burning does. It produces almost no smoke, and ideally it only produces carbon dioxide.
You "sacrifice" about half (or a bit more) of the wood to heat the remaining material, that becomes the biochar.
I made the simplest retort, and I have done three burns, so far:
The inner barrel I have is ~20 gallons while ~30 gallon is a better size. At first, I had too much primary air, and it burned very hot, but didn't last long enough. My third burn was ~3 hours, which is still on the short side, but I got good quality biochar, with a glass-like sound - see the video.
My challenge now is how to use the heat - I want to collect heat from the flue, with a water jacket, and I'd like to try a Peltier to generate some electricity. As you can see, I tried cooking on it, but I need to get solid contact surface - and I now have a heat shield to keep my legs from scorching!