Pretty sure he meant worst case in terms of reducing oil reliance, i.e. if use and demand can't be reduced to maintain existing supply then biofuels have to make up the difference.

Some biofuels, particularly corn ethanol, are bad for the environment. Given the low return on energy invested for corn ethanol, it's functionally a way to launder fossil fuels (e.g. diesel for tractors, natural gas for fertilizer production, coal or gas for distillation) into "green" fuel. With a nice side effect of further degrading valuable farmland.