This decision tells us nothing about the safety of the Mercedes system compared to its competitors. All it tells us is that adding these limitations to ensure the system is only used in the safest possible scenarios makes taking over liability more reasonable. That isn't surprising. Their competitors' systems are also very safe if used in this manner. The only difference is that the competitors are not satisfied with releasing a system with enough limitations that it only works in stop-and-go highway traffic in clear weather. It is that added functionality that is more dangerous and the reason other manufacturers don't take on liability.
Odds are the marketing and accounting wings of Mercedes had just as much if not more influence on this decision than the tech team.