The fruit tree numbers are a bit silly; land isn't free, they should really be including the rent/mortgage cost of the land too. The returns would be less impressive at that point I'd imagine.
Almost every property you purchase in the United States will have setbacks and easements on it which make a large portion of the land unbuildable. For example, a typical suburban lot in my area will have a 5 foot utility easement along the sides and back, then a 20 foot front building setback. It seems to me that you're decreasing the effectiveness of your mortgage payment by not doing something with that area like gardening on it.
Just be careful where your property lines are when you plant your trees. Whether a tree is on one property or another seems to be a common dispute. However, if you're giving your neighbor several pounds of free fruit, they may be more amicable to a tree growing a foot or two on their side.
Since it's assumed that you already own the land in question, they're only looking at marginal cost of the tree itself. I don't see a problem with this.
Apple trees can be grown in pots. Also small fruit bushes (blueberries, blackberries, raspberries). Also beans. You can put these on roofs, fire escapes, windowsills and so on which are effectively underused and thus rent free.
