From my understanding, South Korea has never been closed to US tourists. Just that you are required to apply for a travel authorization through the K-ETA site and do a 14-day quarantine in a gov't facility at your own expense, which I hear is either dropping to 7 days if it hasn't already or might possibly being removed completely soon if you're vaccinated. IATA Travel Centre shows that US passport holders are eligible to enter with quarantine.
From the K-ETA website when USA is selected:
Length - Up to 90 days
Applicant:
- A person who does not have a valid Korean Visa
- A person intending to enter the country for the purpose of tourism, visiting relatives, participating in events or meetings, and business (other than profitable activities)
- A person who plans a short-term stay in Korea for business or tourism purposes
From your website:
Passengers allowed to enter: Korean nationals, ABTC / APEC CARD, holder of Residence card / Overseas Korean resident card,
holder of a valid Korean visa (including K-ETA for those of eligible nationals etc) * <- it's likely that is meant to say nations, nationals doesn't make sense since they don't need a VISA