taken from the treats page:
"Do not count on your chickens "knowing" what is bad for them...also do not count on these "toxic" plants immediately being identifiable by finding a dead bird the next morning...usually it is a slow process damaging organs , inhibiting the ability of your bird to utilize the nutrients in their feed, etc."
I agree with this. Just look at all the drugs that have made it through all the trials and been used for years before someone discovered the dangerous side effects. (I know, drugs and food are not the same, but the point is you can take something for a long time and not see the harm being done immediately.) My position is that I'm not going to get stressed out about what they may come across while ranging in their large pen, but I'll try not to give them anything on the "no" list on purpose. (and follow the links on the list to a much more comprehensive "no" list)
What I'd really like to see someone come up with is a list of toxic things (different lists for different species) organized by how toxic they are - for example, if you look at almost any list of plants toxic to cats (which I know more about than chickens) you'll find poinsettia and lilies (among many other things) but the reality is poinsettia is not toxic, it just contains an irritant that may make them vomit - not fun for them or you, but not really dangerous - whereas a small bite of a lily, or even grooming lily pollen off their fur, can cause irreversible, often fatal, kidney failure. It would be nice to know which things to get worried about immediately, and which are less of a problem if the exposure is minimal and occasional.
Ok, as to the original question - could it be what they mean is that feeding lots of fruit to layers causes them to eat less normal food, and therefore get less protein, since fruit has almost no protein? I've even seen that you should be careful how much grain to give layers because it has less protein than feed (and grain has a lot more protein than fruit).