I thin kyou will find that once they finish their current roudn of hiring, they will have built a business that can support 1m customers. At that point, expenses wont grow (a bit of server and bandwidth) so each additional customer will be pure profit (or pure loss reduction).
Compare to a car company that wants to build a plat to supply 1m cars per year. In the first year, they only have 1000 customers. They have to build the whole plant however as you cant give those first customers half a car.
The difference between XRO and the car example is that for each new customer XRO has no cost but the car company has direct material and labour costs.