As a connecting ICN passenger (KE > KE) in 10 days, I just received an email from Korean Air about the quarantine. Makes no mention about connecting passengers, though I would be astounded if they are including them too. I'll assume quarantine is for those passing through immigration / customs and officially "entering" the country.

They could have saved themselves countless phone calls & other contacts, along with passenger anxiety, by just addressing the subject of connecting passengers in their email.