In the context of plugin code, "this" is a jQuery object. In event callbacks or operating on DOM elements, "this" is a plain old DOM object.

Saying $(this).somejQueryFunction() is defensive programming, it ensures that your object gets jQuery'd. If it's already a jQuery object, it's not expensive to do. This is generic advice for all the various kinds of 'this' that you'll encounter in javascript, not the specific case mentioned in the linked article.

It does seem a bit nit-picky. I would expect that right away in the JQuery call tests if it's already a JQuery object and short-circuits. It would save you one call and it's not exactly in a tight loop.

Maybe as a sign-post that the person doesn't really understand JQuery plugins very well…

Yea great article. I also had a simliar misconception.