How long did you wait on the page? At first when it loads you can read it just fine, but then an obnoxious pop up appears. There is a little "not now" link on the bottom that will get you back to the post, but I can see how someone would miss that (it's a great example of a bad UI).
I think it’s a good example of UX that encourages visitor to sign up - which is why it exists. A part of what makes it good is precisely what you mentioned: very easy to miss the button