You need more people to manage 1,000 servers than you do for managing just 10. Sure, you can automate processes, but as automation becomes more sophisticated it takes more time and resources to build and maintain. So, yes, the number of devs does not grow linearly with the number of servers, but maybe it grows logarithmically, or even slower. But it does grow.
I've worked with them as an affiliate too - and their XML service, in some cases, is very slow compared to their competitors - like orders of magnitude slow. I hope 40+ engineers can help them catch up with the rest. Although 40 seems like a crazy number.