That doesn’t make any sense to me, and it’s been awhile since my biofuel class but I don’t think that’s true.
The carbon in the combustion product comes from the biofuel, the biofuel comes from plants, the plants get the carbon from the air. Therfore, by definition, every gram of Co2 released by combusting a biofuel came originally from the atmosphere.
It has to be neutral, otherwise where is the magical extra carbon coming from?