The reason why you aren’t supposed to do that is because fruits and vegetables have different ratios of carbohydrates and protein.. Fruits are higher in simple sugars and are broken down sooner and vegetables tend to be higher in protein, so they take longer to digest.
It probably won’t hurt, it’s just no optimal to mix them.
Everything breaks down at different rates in your system. Even the same base food will break down at a different rate depending on how it is processed
You think you should only eat a single food at a time?
I have to admit that is one of the odder diets I have ever heard of but you do you I suppose.