Definitely a very valid point that is worth reiterating! Especially true for consumer products which are older than 3 months (by then they should have learned that this is best practice) and which have lots of similar competitors (which, at least in my case, lowers the threshold of "screw it, I wanted to try this other product anyways" if something doesn't work).
I think one part of this is dispelling the myth that telling the user which of the fields is wrong is a security flaw. You can already figure out if a username exists by trying to create it on the registration screen, so saying "Your username and password do not match" does not provide any extra security, and just frustrates users.
The author has described anecdotal experience with login screen of some product but he hasn't actually stopped using that product. So what is this to do with user retention?
