Bard image results include previously generated StableDiffusion images

I think he's misinterpreting what is happening. Bard can now return images that Google crawls from the web. Bard didn't generate the image. Its returning the image that he used Stable Diffusion to generate because he uploaded it to a site that was then crawled. The Prompt Hunt badge is acknowledging Prompt Hunt as the source. Thats also why the same prompt returns in the same image.

Bard was updated to do this on 5/23/23:

2023.05.23 Bringing images to Bard

What: Starting with English responses, Bard can now bring in images from Google Search, so you can get helpful responses with visuals. You can also ask Bard for images directly. Bard will show a source for each image. Why: Images can help you communicate your ideas more effectively. They can bring concepts to life, make recommendations more persuasive, and enhance responses when you ask for visual information.