What do you mean by "maybe" or "light"? That's an explicitly permitted activity:
> A B-1 visa may be granted to specialized workers going to the United States to install, service, or repair commercial or industrial equipment or machinery purchased from a company outside of the United States, or to train U.S. workers to perform such services.
https://es.usembassy.gov/visas/commercial-industrial-workers...
Modern factories are filled with machines the size of buildings, making that installation sometimes hard to distinguish from the forbidden "construction". It's possible that some of those Koreans were unequivocally on the bad side of the line, but I see zero possibility that the agents could have meaningfully assessed that in the time between beginning the raid and taking the workers away in shackles.
I feel like installing equipment is widely considered to be an illegitimate use of B-1 visas, despite this explicit guidance. I don't understand why. I see from your comment history that you were a US diplomat. Is the internal guidance you received different from what's published?