This problem is often referred to as
user enumeration, which seems like a misnomer to me.
Whatever you call it, it's good to keep in mind that Google, Apple, Facebook, and Twitter are vulnerable.
The most interesting facet to user enumeration is privacy. If Alice knows Bob's e-mail address, is it right that she can easily check with Carol's web system to see if Bob has an account with Carol? This is like calling a hotel and asking if a certain person is staying there. It leaks information confirming that two parties have a relationship.
What makes the privacy aspect especially interesting is that user enumeration is a bigger problem for small websites than for the Googles and Facebooks. This is because it's not much of a privacy breach to be able to test if a given e-mail address is on Facebook; big whoop, almost everyone is. But it's a much bigger breach to be able to test if a person is a customer of an illicit service. Most systems sit somewhere between those two extremes, and each needs to decide whether to prevent user enumeration.
The main point the author tries to make is quite matter-of-fact: If you are going to try to prevent user enumeration, then you have to do it for real, which usually results in less friendly account creation and password recovery systems.