Order fulfilment companies are also third party companies who store stock, and will pack and ship orders you’ve processed to the end customer. The difference is who owns the stock.
With dropshipping, you, the merchant, don’t own any stock. Nada. Not a box.
With order fulfilment, you own the whole lot — you’re just using another company to store and ship it for you. You’re in charge of managing your own stock levels, and putting in the money to stock up in the first place.