The car can only read stop and white speed signs. It can visualize markings on the ground and perhaps some are being read today like railroad crossing and turning arrows.Are you thinking it can "read" the words "STOP" but not "except right turn?" It doesn't need map data to indicate it's a free right turn to pick up on visual cues of lack of stop line and extra signage next to the stop sign, etc. There's other examples of 12.x ignoring plain stop signs and only yielding when making a curving right turn, so end-to-end behaves quite differently from 11.x, which would be much more likely to slam on brakes if it realized there was a stop sign it needed to obey.
12.3.x and the driver should have stopped because they were going across an oncoming traffic lane to correctly follow the signage, but there also was no oncoming car to interact with either. A disengagement probably would have helped improve future 12.x to better understand when it's correct to maintain speed versus stopping.
There are many signs the car cannot read and I suspect will be quickly added now they finally reached end 2 end.