Customers have been immunized to full screen modal dialogs, don't read them and close immediately, this represents obstacle to subscribe to their newsletter. In order to remove that obstacle, they bravely decided to remove close button, so that potential customers have no other way than "The Right Way", are not distracted by close buttons anymore and can successfully subscribe to their newsletter, which helps with growth and widens funnels (that's good, right?).

Oh good, I thought there was something wrong with my browser that was hiding a close button. That's a very strange design decision.

Very strange, and a bad idea to boot! It is distracts from whatever point they are making in the article. The medium is the message!