As of now, the seismic coverage on this earthquake is limited. However, data from various sea level buoys indicated a significat tsunami wave. In addition, the wide spread aftershocks in the region indicate a much larger rupture than a moment magnitude of 7.5 would anticipate. 
Thus, a larger moment magnitude was used to simulate the tsunami. It was estimated that the total earthquake rupture which caused the tsunami was equivalent to a moment magnitude of about 8.3. The earthquake rupture model used to estimate the tsunami had a total rupture length of 210km and a maximum displacement of 20m.

The resulting tsunami pattern corresponds well the buoy data found here SEA LEVEL STATION MONITORING FACILITY (ioc-sealevelmonitoring.org):

• King Edward Island: 0.7m 
• Syowa Station (Antarctica): 0.6m
• Marion Island: 0.4m
• St. Helena: 0.4m
• Stanley station (South Georgia): 0.3m
• Takoradi (Ghana): 0.2m
• Reunion Island: 0.2m
• Palmeira (Cabo Verde): 0.2m 
• Base Prat (Antarctica): 0.1m
• La Gomera (Canaries, Spain): 0.1m

Right now, it is unclear how the actual situation looks like. And the current tsunami simulation is an educated guess on what has happened on August 12th, 2021 in the Souther Atlantic Ocean.

There is only one historic very large earthquake known from the South Sandwich Islands. In 1929, a Mw8.1 earthquake rupture its northern interface. Source: USGS

Update, August 17th, 2021:

The earthquake has been updated to Mw8.1 and an epicenter further south which corresponds close to the estimated tsunami earthquake simulation above. Reanalysis of the seismic data has revealed that the original Mw7.5 earthquake was succeeded by a much stronger Mw8.1 which ultimately triggered the tsunami and the main aftershock sequence. The event details have been adjusted to represent the tsunami-triggering event. 
Source: USGS