That depends on what you mean by "wide-scale". I agree that it would be impractical to kill 10,000 people this way, but you could do a bang-up job of killing just about everyone in an enclosed space like an airliner or a moderately sized building (via aerosol), or everyone who was fed from a given food source (via powder in the food).
Because symptoms don't start showing for several hours after ingestion, no one would notice anything was amiss until everyone had finished eating, even if the eating is done in shifts as at a mess hall. Think of all the corporate and government buildings which feed most of their occupants in a given day via a large cafeteria.