No, they said "...the law can stop you from repairing...", which is true.
"I also wouldn't be surprised if (much like cars), tractors generally only need mechanical repairs over the duration of their lifespans."
I know little about tractors, but your statement is not true for modern cars. Often repairs cannot be completed by mechanical means alone, and require interfacing with or modifying the software in the car. Even if you replace a broken airbag, the light will stay on until you take the car to the dealer to have it reset.
Even if your statement were true, who gets to decide what repairs are required over the life of the car/tractor? If not the owner, then why not?
But you'd be wrong. Take a look at the top comment here[1]:
A recent BMW, in which all I did was replace a dead battery, required a trip to the dealer to "accept" this new battery.
So yes, the ECU software can really do prevent you from repairing any mechanical part of the machine, as long as the ECU has a monitoring sensor for that part.
I also wouldn't be surprised if (much like cars), tractors generally only need mechanical repairs over the duration of their lifespans.