If they had said that they have concerns about safety, and requested an urgent (possibly remote) discussion, the employer would have more options. By telling the employer that they were leaving immediately, and not coming back until they felt something had changed, they force an immediate decision of either treating it as a resignation, or accepting and condoning unilateral actions by employees.
Strikes are definitely ultimatums, though they generally have clearer objectives, and are usually preceded by a series of discussions. Strikes are not conducive to good relationships.
Read this again, and comprehend that the HR unilaterally and knowingly subjected them to risk of contacting the virus.
Subsequent self-quarantine at home benefits the company as it reduces the chances of spread. It was the right thing to do, and there's not much to negotiate here given the initial screw-up.
I am not making a judgement as to who is at fault for the situation. I am only stating that the decisive move was the grandparent's e-mail, which left the employer only one rational option (that I know of).
What does Samsung stand to gain from this? They lost an employee (which will cost them in recruiting when they open up again) and gained bad reputation (and a disgruntled former employee).
This is power play that hurts everyone.
You do know that frequently the only reason these "discussions" are able to be had is because of the threat of a strike? What if the GP had these "discussions" and was told to go back to their desk and STFU? Then what?
Strikes are actually the result of employers abusing employees, which is what creates the bad relationships. You sound as though you believe the other way around, that strikes create bad relationships.
Edit: speaking of "ultimatums," what do you call a strike? Is it only an "ultimatum" (thus unacceptable) when one person does it, but not when a large group does it?