I don't know if it was or not. Maybe they just decided to honor it because he had no other will and the people involved were amicable and respectful to his wishes. In any rate it is not really a legally binding document, and if someone wanted to challenge it, they could.
The farmer's will should be accepted as a legally sound will if the farmer resides in a state that allows holographic wills. A holographic will is a will that is entirely handwritten and signed by the person making the will. If the farmer lives in a state where handwritten wills are legally acknowledged, then his will should be legally recognized.
Holograph wills are valid in Saskatchewan, with one of the most famous that of a farmer who was pinned under his tractor and scratched his will onto the fender. The Court admitted the tractor fender to probate .Where there’s a will, there’s a way...
But actually, in the case of a Saskatchewan farmer, the reverse is closer to the truth. Back on June 8th, 1948 Cecil George Harris was working his fields when he became caught underneath his tractor. His left leg was pinned under one of the tractor’s wheels and he was unable to free himself. Fearing the worst, Cecil used a small knife to carve out his last will and testament on the fender of the tractor. It read simply “In case I die in this mess, I leave all to the wife. Cecil Geo Harris.”
Although he was eventually found and freed by neighbors, he died of his injuries two days later. And although he never mentioned having made his will as described, the writing on the fender was discovered. Once it was established that the handwriting was that of the deceased Cecil George Harris, the will was admitted into court as a valid holograph (handwritten) will.
The farmer's will on the fender of his tractor should be accepted as a legal document because it expressed the will of the farmer. Likely, it will come down to the laws of the state the farmer lived in, but in many states, a person can make a will just by writing it out and signing it. If it appears to be valid it should stand.
It should be respected, as it is his property and if that's what he wants it's his right to do what he wants and should be respected, as a human with natural God-given rights. As a "legal" document? Simply put, yes it should. But it gets into a much deeper issue when you look at it because I'm not a statist. Legal authority should be made under natural law and Gods laws. If it differs, then simply put, it's wrong.