I think we can safely assume each datacenter holds a dedicated load-balancing server. Let's assume they have only one of these at each datacenter, it gives us 70000 / (4 * 12) = 1460 requests per second per CPU, which is in line with reliably handling over 10,000 request per second of live traffic to WordPress applications from a single server.

and

In April 2008 Automattic converted all WordPress.com load balancers from Pound to NGINX. , which points to the fact that they're only talking about load balancers and they have several of them.

This link implies that one NGINX load-balancing instance cannot handle 70k r/s.