SynthID-Image is not the picture itself (it's applied post-hoc—which is to say, after the image is generated), but it is a very, very robust watermark that can stand up to large amounts of compression, text overlays, cropping, and a bunch of other stuff.

As to the graphic on the shirt: there is no way in this image to tell if the graphic is real, as far as I can tell, and a quick search found only shirts, but if anyone can find the original image and run it through OpenAI's SynthID detector...