No, it's definitely a dark pattern. Everything about the page suggests that pressing the "generate 4 images" button will generate four images. And then it does

No, it's definitely a dark pattern. Everything about the page suggests that pressing the "generate 4 images" button will generate four images. And then it doesn't and tries to lever that anticipation to get a sign-in. It ends up just making users angry. If you need a sign-in, e.g. to prevent abuse, you could start by explaining that a sign-in is required and why. At least then people won't be angry when they are ambushed by it