Yeah, it has to be that or something very similar.
I am not an EE but I have slept in some motel's so here goes... electricity moving in a conductor creates an external field which will induce electricity in an adjacent conductor, that's how transformers and some meters work. You with me so far? OK, so if you drag the weding cables over and around any wiring on the tractor you may be inducing voltage within the tractors wiring harness while you have an arc going, battery or no battery. Now, it's difficult for me to believe that welding cables properly routed could cause any problems. If the tractor has bits and pieces that are that sensitive they won't last anyway.
Given the proximity of fuel, plastic and battery gases then, yes, it's a good safe idea to weld off the tractor when possible. I wouldn't even bother to disconnect the battery on mine. In fact I think I will try it just to see what happens.
Later, I'm going back out to sleep at a motel to see if I'm correct.
All the Best,
Martin