That is not true.

There is a process, it's usually tedious but it exists. I did it for Singapore, the US and Israel. They mostly took multiple months but I never wanted to take any chances. For the US it was a "B-1 in lieu of H-1B" visa for example.

Attending a conference is something different than what these workers did. There are rules around what a "business trip" is and what is not and what "work" is.

There are visa types specifically created for this sort of situation (e.g. E2 visas). But those visas are only available to certain nationals and South Koreans are not among them, which is very stupid given the strong commercial and strategic ties between the USA and South Korea.