I'm surprised that the battery was able to start it while in gear.
Gas or diesel, doesn't matter.I know that a gas engine will with no problem, yet my old 275 would just click if I had it in gear, no matter what I put in it for a battery.
Ag tractors don't have a seat switch.
Many guys start their tractors that way. Not a problem if you pay attention. Maybe not a great idea for an 86 year old.