I'm an American living in Berlin, and I think the problem here would be more of a cultural one than a legal one. Germans just have an expectation that once hired they will be given every chance to succeed, to the point that firings seem to come across as a shock or an implication of major wrong-doing.

"Hire fast, fire fast" is already ingrained in American startups, so I think the policy outlined in this article would be easier to adopt in the US. Americans in general view jobs as more transient than Europeans do.