According to this source, it is claimed that (emphasis is mine)
At Arbikie Distillery in Angus, scientists claim to have developed the world's first "climate positive" gin. By using garden peas instead of wheat, the company has eliminated the need for carbon-intensive fertilizers and can produce animal feed from the leftover material. Taken together, this gives the drink a negative carbon footprint, Arbikie says.
I am having trouble understanding how the act of producing animal feed from the leftovers of a process results in a negative carbon footprint.