For 99.9% of people, if they don't store their passwords on their computers, they're either going to use weak passwords, reuse passwords, or both. This article is like saying that for safety, you should drive long distances instead of flying because planes sometimes crash and kill hundreds of people at once.
People are bad with passwords, so while I understand that a single key to the kingdom is a nasty idea, it's better to have most people trust password managers. Better 1 bad password in 1 service than 1 bad password in 20 services, lower attack surface, though the gains are much greater.
I think a better recommendation than "don't use a password manager" might be something like "don't use a password manager for email/bank accounts, do use it for everything else."