Like anything in law, it depends on the details, context, case law, and possibly future litigation.

CPB's Q&A on permissible B-1 visa activities (PDF): https://www.cbp.gov/sites/default/files/documents/B-1%20perm...

"If the contract of sale specifically requires the seller to provide these services or training, and you possess specialized knowledge essential to the seller's contractual obligation to perform the services or training it may be permissible for you to perform these services. In addition, the machinery or equipment must have been manufactured at a location outside of the United States and you may not receive compensation from a U.S. source."

Given how vague the reporting has been, we don't know basic facts like what the workers were doing, what the agents saw, what types of visas they were on, etc.

This PBS article quotes a local labor union leader who claims "unions that are part of her council believe Korean workers have been pouring cement, erecting steel, performing carpentry and fitting pipes." https://www.pbs.org/newshour/nation/attorney-says-detained-k...

Obivously the person quoted has an agenda and didn't actually witness those activities - so we just don't know. That being said, if that description is accurate, in general that kind of activity would not be appropriate on a B-1 visa and wouldn't qualify for an L visa.

But again, it depends on the details. Maybe the cement base is some special blend for certain equipment, maybe "erecting steel" involves highly specialized welding techniques, maybe this pipe fitting involves specialized high-pressure ratings outside the norm.

When I was a diplomat, our internal guidance (at least, what I was privy to) was never different from public information, just more detailed.

I'd be suprised if the corporate immigration departments of Hyundai or LG messed up this badly. But I wouldn't at all be surprised if some no-name subcontractor decided to play fast and loose with the visa rules to win a contract with a low bid.