It's super impressive to me, and getting this kind of fidelity is honestly brand new. You can include an image in an AI image generator to serve as an outline. In this case a blank image with bold text. It'll fill in the prompt using the reference as best it can (in this case, a bunch of kittens/puppies).

Prior to a few weeks ago, I'd only ever seen this using a posed figure--basically a stick figure--to generate the prompt's subject around.

I think the way stable diffusion works makes this kind of message imprinting quite easy to implement.

the wikipedia article has some good pictures

I would imagine it works by inserting the text on some of the first steps so the ai has the text as it's seed instead of just random noise.