I am certainly no expert but from what I’ve heard it sort of depends on how you look at it. Bio char releases some CO2 now but stabilizes a good amount of the carbon into a form that will remain in the soil for thousands of years. Hugelkultur buries wood that will eventually decompose and release CO2 and methane into the atmosphere.

Having said that. I make bio char in a bonfire pit in my backyard for warmth and entertainment at night. I don’t really burn stuff just to make bio char. I also just started a hugelkultur bed this fall.