Psst. Don't tell anybody, but the answer to your question is in the second paragraph in the article:
Similar to outpainting—an AI imagery technique introduced by OpenAI's DALL-E 2 in August 2022—Midjourney's zoom-out feature can take an existing AI-generated image and expand its borders while keeping its original subject centered in the new image. But unlike DALL-E and Photoshop's Generative Fill feature, you can't select a custom image to expand. At the moment, v5.2's zoom-out only works on images generated within Midjourney, a subscription AI image-generator service.
Her mutant power certainly isn't witty one-linerd