The US has a history of being an unreliable ally. Just ask Afghanistan. Or Vietnam. So it’s no surprise that South Korea is having some doubts about the commitment that the US has made to defend them. So when Biden announces that US nuclear armed submarines will be docking in South Korea is a sign that the US will extend its nuclear umbrella to that country, I’m sure they are taking it with a HUGE grain of salt.
It’s the same reason why the Philippines says that US bases in their nation can’t be used for offensive operations.