AI does not specifically have trouble generating salads, but it is true that AIs have occasional difficulties with specific human extremities in making them look realistic. The original post that you cited talks specifically about the AI's incapability to make teeth and hands look real, but the salad is also a part of this equation and the question of why the generated image was not realistic.
AI is in its infancy as a technology, so it is understandable that there are some glitches as they are trained with more information and knowledge. AIs are trained with real pictures and databases in order to create more realistic images, and they pull from this information to generate images that otherwise would not exist.
Miles Zimmerman, spokesperson of Stability AI explains in PetaPixel's piece Why AI Image Generators Can’t Get Hands Right that "Part of the reason AI image generators do so badly with hands is that in the datasets used to train the image synthesizers, humans display their extremities less visibly than their faces." He went on to add also the dilemma of positioning within reference images, saying "Hands also tend to be much smaller in the source images, as they are relatively rarely visible in large form." With this line of thought, any picture of a human eating a salad that is fed into AIs presumably feature salads, but not as the focal point. Because it is not as large or detailed as the human faces featured in stock images of people eating salads, AI oftentimes gets confused.
Salads were not the main issue here, but your original claim still initiates curiosity as to why AI generators are better at generating certain things over others. I would rate this claim as misleading as there is more going on here than just salads being a tough point for AI generation.
AI original source: https://pettingzoo.co/@gronk/110070382666359354
PetaPixel source: https://petapixel.com/2023/03/02/why-ai-image-generators-cant-get-hands-right/#:~:text=Why%20is%20AI%20so%20Bad,AI%20spokesperson%20tells%20BuzzFeed%20News.