The warning concerns "dense" food products; things like big slabs of cheese, large quantities of solid chocolate, smoked meats, etc. On an x-ray they are solid, like something lead-shielded, so they have to physically see what they are.
Don't put perishables of any kind (or liquids in glass bottles) in checked baggage. If the glass breaks your other things will be ruined, as will those of other passengers. If your bag is lost, perishables may rot before you can reclaim it, again ruining the rest of what is in the bag (not to mention the lovely probability of maggot infestation.)