German stabbing suspect is 26-year-old Syrian man who admitted to the crime : europe

The fact he submitted himself on his own to the police and couldn't provide substantial evidence so far that linked him is strange for an IS member.

This isn't entirely inconceivable. Mentally ill people sometimes confess to high profile crimes they didn't do for mentally ill reasons. There was a really good scene in Hawaii Five-O where in the middle of the episode some guy suddenly walks in and confesses to the murder, but he doesn't know the details of it. And McGarrett realizes that he's just a very lonely sad person who hasn't had any friends since he moved to Hawaii. This happens often enough that when cops are interrogating people they'll ask leading questions to see if the suspect is just confessing to everything they're accused of.

But it's pretty darn unlikely that someone in that condition could fool the cops in a major terrorism case long enough for them to release his name to the public.