Similar but different.
The working "current" while welding is an arc, with heat, melting the metal.
Same current flows through the welding leads, ground clamp, and so on: no arc, not much heat, no melting.
The bolt in the hole is already doing a good enough job conducting that current (similar to the welding leads) it doesn't arc between the threads and weld them together, at least not in my experience.
