I'd read it as implying the opposite: that there are 40,000 moths out there who will wind up in a bear's stomach tonight. Like "some 40,000 people per day are hit by a car"; you wouldn't take that as meaning one car hit 40,000 people.
"That means that 20,000 calories of just moths per day can be consumed by "A" rock-turning grizzly bear."
How can you possibly read this and think this was about the whole bear population?
"Hit by cars" means multiple cars. "Hit by a car" suggests one car. We're often sloppy, and often there's an implicit understanding that contradicts what's said directly. Clearly this is silly, one car can't do that so we interpret.
Neither figure seems particularly compelling here, of its "eaten by bears" it seems way too small if it's "eaten by a [single] bear" it seems too big.
