What's threatening to Merkle's government isn't the incident. What's threatening to Merkle's government is the fact that they deliberately attempted to suppress the story that a Muslim beheaded a baby. In fact, you proved it by confirming that the story originally occurred back in April. And, the Merkle government brought the full force of the law on not only the person who videoed the incident, but the blogger who mentioned it.
I expect the Germans can access British newspapers on the internet and a fair number of them probably do. This story would have been known in Germany and the fact that it was suppressed there. There have been many negative aspects of migrant policies and much dissension by some in her ruling coalition, and many recent meetings to try and develop a new strategy.
I don’t think this is the seminal moment the tweet seems to suggest, but just one of many incidents and there have been no dramatic changes yet.