The su command is correct for that purpose, as is sudo. This is kind of a subtle point that the article makes—the problem is that “su” is not good for dropping

The su command is correct for that purpose, as is sudo. This is kind of a subtle point that the article makes—the problem is that “su” is not good for dropping user privileges in general, but it does work if you are a user trying to spawn an interactive shell as another user. Keep in mind that you are giving control of your terminal to that less-privileged account, and that this is a potential point of privilege escalation.