It looks like they are detecting crashes of other cars caught on camera, not crashes of the car with the camera.

They are not crash-testing automobiles, so an accelerometer won't help. They are looking at dashcam footage from a library of that footage.

The key factors are 1> reducing BOM and 2> working in cases where you have whatever data are available (perhaps a phone or CCTV footage or whatever).

As it happens most if not all dash cams have accelerometers.