Requires login. Saying that it requires login before someone spends time to think up a description for an image would have been nice (because I won't sign-in to try something anyways). Edit: rephrased to remove overly negative language.
I don't see this as baiting. The link to a tool page was shared. It's transparent on the homepage and in the header. The text to image page just shows how it works without the need to create an account.
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