This is an example of where the person who came up with a term screwed up. You're talking about how you sign your email, but the post is talking about the signature line (or lines), which are not a substitute for signing your message.

For personal emails I don't use a signature, I just sign it. For business emails I sign the email and have a signature line with my name, title, business group, phone numbers, and group website. --This information fits on one line and is easy to refer to or ignore as per the recipient's needs.