Fungal and Bacterial growths, that would be harmful to your fish, would only be introduced if your food was contaminated.
That is like saying, "Don't go into that brand new, never-before-used, hospital room, because a pneumonia patient will someday be in there."
It is misleading information to say the best, and absolutely incorrect to say it more accurately.
You will always introduce some bacterial growths to anything you come into contact with. Believe it or not the human epidermis is quite a filthy place if you could see under microscope, but 99.999999% of these bacterial growths, or cultures, are not harmful to us or fish.
True fungal infections are much less common than people would have you believe. What happens more times than not is that it is a misdiagnosed bacterial infection, or simply, a bacterial growth or culture.
So basically the information is correct that food will introduce bacteria, but it is misleading in that it introduces NON-HARMFUL bacteria, and it is incorrect that it introduces FUNGUS (unless it has been pre-contaminated post-production).