When I’m looking at ÖPNV, I see that there are many numerous yellow area that connect different bus stops together, like this: 
(captured from around the region of Tai Koo station of Hong Kong MTR system), however I cannot find any stop_area-tagged area in the region which was supposed to be what would indicate something being a stop area to ÖPNV. In such case, how ÖPNV identify the stop area relationship between different stops?
As far as I know both ÖPNVkarte and the “Transport Map” layer on the main OpenStreetMap page group together stops with the same name. It’s something I have noticed where I live, where it gives rise to weird-looking shapes that don’t have any relationship to the real world, like this: https://www.openstreetmap.org/node/4464504995#map=19/36.72362/-4.37714&layers=T
In your example I think that might explain the Kornhill Plaza and Cityplaza areas, which appear on ÖPNVkarte. But it doesn’t seem to explain the yellow area that includes Block 2 Kornhill Gardens (which in this case doesn’t show up in the Transport Map). I guess there is something else going on there but I don’t know what.