Carbon-negative is a long stretch, it's just using waste material that is usually used as fuel. It's at best low-carbon compared to concrete, which honestly is already a good thing.

At around the same time, Meyer learned about the large amount of waste lignin that is produced every year, primarily from pulp and paper processes, which is also expected to be produced from biorefineries in the future.

[...] During the production of pulp and paper products, roughly 100 million tons of lignin are produced annually as a waste byproduct and subsequently burned as low-value fuel.

Meyer saw lignin as a polymer that could be used as a material instead of a fuel and sought to crosslink it like an epoxy resin. Using lignin allowed Meyer to sequester CO2 captured from the air in the form of biomass that would otherwise be burned.

(Emphasis mine)