I believe they're just referring to having several completely-independent postgres instances on the same host.
In other words: say that postgres is maxing out at 2000 conns/sec. If the bottleneck actually was fork rate on the host, then having 2 independent copies of postgres on a host wouldn't improve the total number of connections per second that could be handled: each instance would max out at ~1000 conns/sec, since they're competing for process-spawning. But in reality that isn't the case, indicating that the fork rate isn't the bottleneck.