Using car batteries for various types of welding is common hobby practice
Something tells me most hobbyists are not doing their car battery welding on-the-fly in -60F weather in the middle of Sibera with the stakes of failure being freezing to death though.
I inferred that he was simply saying the fact of using chained car batteries for welding is hardcore, which I obviously think is not as I've seen it done by many people in many places.
As for the situation in this context though - sure, that could be said to be hardcore.
You're right, car batteries are a very accessible source of high-current DC power. It's especially common for budget DIY spot welding.
It is, it was likely some big thing, like strut support, not small hobby things like jewelry.
Arc welding structural steel with lead-acid batteries is well-established among hobbyists. Here's somebody who reinforced their truck bumper using six 6-volt golf cart batteries, jumper cables, and filler rod, in 2008: