I don't know if their stack allows for it, but the number of concurrent connections is limited by unique 5-tuples (which is what the host uses to identify the s

I don't know if their stack allows for it, but the number of concurrent connections is limited by unique 5-tuples (which is what the host uses to identify the sockets), not just source ports. There is presumably quite a lot of entropy available in destination IP & port to enable far more than 2048 connections per server. But it's hard to be deterministic about how much, so they might be choosing to ignore this and use the lower bound.