So then you think welding something else in the garage with your car parked in it is going to fry the car? I have both a Tig welder with high frequency start, and a industrial type Mig welder with in 4 feet of a simple old boombox that's on while I weld, never any type of interference on the radio while welding. But did have a old bad florescent light that gave interference on the radio.But there is still the matter of EMI. Depending on the type of welder it could be inducing high frequency into wires if welding leads aren't routed sensibly. My hypothesis about leaving battery connected or shorting the leads together is meant more to address that.
Welders made today have to pass federal standards as to radiated emissions. as to welding leads I've never routed the leads through anything when welding, so can't answer that one.