OK, I found the original story that Bloomberg is reporting on. Reading it via Google translate to Japanese (because Korean to English sucks)
손님 27명 감염, 직원은 멀쩡 파주 스타벅스 미스터리 천장형 에어컨 밑에 앉은 확진자, 방문 2시간 30분새 줄줄이 감염 2층 화장실 잠깐 쓴 초등생도 걸려 KF94 마스크·위생장갑 착용한 직원들은 수시로 오갔지만 무사
www.chosun.com
Here are the details:
On the 8th, Patron A (female, 30s) enters the location and orders a drink on the 1st floor before going up to the 2nd floor seating area. The next day, she began feeling Coronavirus symptoms. On the 11th, she took a test. On the 12th, she got back a positive result.
During the 2 hours and 30 minutes she was in the Starbucks, they believe 27 other patrons also became infected. Almost all were on the 2nd floor, but one grade school aged child who became infected only went up to the 2nd floor briefly to use the restroom.
The 2nd floor space had 6 air conditioning units that were in operation circulating the air to keep the area cool. The article also claims that air conditioning units like this have been shown to dramatically increase the range and spread of infectious particles.
It does say that the 4 employees who were not infected frequently visited the 2nd floor space to clean up.
The article does not, however, say how many people visited the Starbucks in the 2.5-hour window (assuming that Patron A was even the one to introduce the virus to the area). That seems like a critical factor when determining if the masks did anything.