The bit about masks is my own conclusion based on a lot of different local sources.
The closest thing to a comprehensive source about the distribution of infections in South Korea that I can find is [1], which still contains too much Korean-in-a-canvas that can't be automatically translated:
54.6% of cases are related to the Shincheonji cult
0.5+0.7+0.5% various other churches and religious groups
1.3% Daenam Hospital (almost all cases occurred in the psychiatric ward, as well as the attached funeral home where the Shincheonji cult leader held his brother's funeral)
1.2% related to gyms
1.2% related to the call center in Guro-gu
3.6% nursing homes
6.3% related to other mass infections (this includes other call centers, nursing homes, etc.)
11.3% contact with people infected from one of the other sources
3.3+0.6% infected overseas
14.9% other/unknown
Of course this doesn't say anything about masks. But you can clearly see that unlike in many other countries, ~70% of cases in Korea are concentrated in a handful of clearly delineated groups, with relatively little spillover into the general public. It takes a bit of familiarity with the religious landscape of Korea and the workplace culture to recognize that most of these groups are where people don't wear a mask. (As for nursing homes, the elderly often have difficulty breathing through a mask, not to mention they're together 24/7.)
Some people still got infected while wearing a mask, though, so it's not bulletproof. But it seems that the effect becomes visible as a kind of herd immunity when aggregated over a large population.
As for the government not publishing locations visited with a mask on, here's an example from my city's official Instagram account [2]. It says that the infected person's movements on some days are redacted because she wore a mask on those days (among other precautions) and therefore didn't make any epidemiologically significant contact with other people. You can compare this with other press releases by the same account that contain a lot more location data for other people.
The closest thing to a comprehensive source about the distribution of infections in South Korea that I can find is [1], which still contains too much Korean-in-a-canvas that can't be automatically translated:
54.6% of cases are related to the Shincheonji cult
0.5+0.7+0.5% various other churches and religious groups
1.3% Daenam Hospital (almost all cases occurred in the psychiatric ward, as well as the attached funeral home where the Shincheonji cult leader held his brother's funeral)
1.2% related to gyms
1.2% related to the call center in Guro-gu
3.6% nursing homes
6.3% related to other mass infections (this includes other call centers, nursing homes, etc.)
11.3% contact with people infected from one of the other sources
3.3+0.6% infected overseas
14.9% other/unknown
Of course this doesn't say anything about masks. But you can clearly see that unlike in many other countries, ~70% of cases in Korea are concentrated in a handful of clearly delineated groups, with relatively little spillover into the general public. It takes a bit of familiarity with the religious landscape of Korea and the workplace culture to recognize that most of these groups are where people don't wear a mask. (As for nursing homes, the elderly often have difficulty breathing through a mask, not to mention they're together 24/7.)
Some people still got infected while wearing a mask, though, so it's not bulletproof. But it seems that the effect becomes visible as a kind of herd immunity when aggregated over a large population.
As for the government not publishing locations visited with a mask on, here's an example from my city's official Instagram account [2]. It says that the infected person's movements on some days are redacted because she wore a mask on those days (among other precautions) and therefore didn't make any epidemiologically significant contact with other people. You can compare this with other press releases by the same account that contain a lot more location data for other people.
[1] https://coronaboard.kr/en/#source-card-slide
[2] https://www.instagram.com/p/B-Iy3G7lTQs/