Well, for marketing purposes, random doesn't sound as impressive as intelligent. It's a discrepancy but it does appear to be disclosed.

On the page you link to, it says: "Incoming web traffic is automatically routed to web dynos, with intelligent distribution of load instantly as you scale."

When you click on "Read more about routing..." it says: "Request distribution - The routing mesh uses a random selection algorithm for HTTP request load balancing across web processes." https://devcenter.heroku.com/articles/http-routing