This actually seems to me like a perfect solution (from WhatsApp's side). This way as long the user has the same phone number, he/she doesn't have to remember any credentials, which is probably the main reason (or one of the top 3) for people using WhatsApp in the first place.
And as for the "security problem", if someone has access to your phone they can just maliciously use the app itself. I'm not saying that this should just be ignored, but in this specific case the author had probably created the bigger part of the security threat by publishing the article.
And as for the "security problem", if someone has access to your phone they can just maliciously use the app itself. I'm not saying that this should just be ignored, but in this specific case the author had probably created the bigger part of the security threat by publishing the article.