Unix shells are generally not viable access control mechanisms any more
This article could have been published 30 years ago. In professional unix admin circles this was already well known back them. Although I could be misreading it as the article is not very clear. I think this are the points it is trying to make:1. Once upon a time you could rely on the passwd file and shell behavior as an effective means of authentication and access control.2. It has been a very long time since that was an effective approach, for a variety of reasons, and you should not do this on modern production systems.