Well, how Löw got that reputation will be a topic for Galileo Mystery in the future. No one knows, but I'm sure the Illuminati are involved here. As it stands, at some point after Klinsmann said that he'd step down someone came up with the idea that Löw was actually the tactical mastermind behind the NT and this has been repeated so often until everyone believes it. That was probably self-preservation by the DFB, the head coach job wasn't necessarily a coachs' dream at this point and they had to justify Löw.
It's even more baffling considering that his adventures in recent tournaments have been somewhat mixed, but people seem to suffer from selective amnesia:
2008: A decent game vs Poland was immediately followed by an abysmal one vs Croatia. Against Austria, who failed to repeat Cordoba 1978, the team was uninspired to say the least. Two flattering wins vs Portugal and Turkey later, the team went to the final to be outclassed by Spain.
All in all, a decent tournament with a lot of issues being... well, let's call it developable. It was Löws' first tournament and he did a good job here. Judging from results only, it was more than good. Löw talked about the team being in a state of development and he was correct.
2010: Now we're reaching the point, where two matches overshadow everything else -let's not get ahead of ourselves here though. Germany beat Australia comfortably as it should be and expectations went through the roof but well.. then came Serbia and Germany sucked. Kloses' sent-off was a joke but the team sucked nevertheless - at this point it was already clear that Löws' team will continue to struggle against well-organized compact defenses with tight positioning - a flaw the team still suffers from until this day. Then came Ghana and despite the win - the overall performance was shocking.
Despite all expectations, what followed were the best two matches in Löws' career to this day. England couldn't cope at all with the fast movement, quick counters and one touch football and was utterly destroyed in the process. Then came the team that set out to prove that compactness is overrated, formerly known as Argentina, and was steamrolled.
The match vs Spain, well... didn't go so well.
All in all, two matches made it to the collective consciousness masking that Germany had a hard time getting through the group stage. Löw talked about the team being in a state of development but that was already doubtful at this point.
2012: The Group of Death - Germany gets through unscathed. Sure, it included a rather obscure performance vs Portugal but nobody's perfect. Expectation went through the roof again and the team answered by beating Greece.
What followed now though, was the worst match by far in Löws' term: Tactically inept, completely anemic performance with an über-flattering result vs Italy. The public blamed the players because they didn't sing the national anthem, Löw blamed Hummels. No one blamed Löw. Löw talked about, you guessed it, the team being in state of development, which was by now proven to be incorrect.
2013: The team is again in a state of development and Löw and his gang including MC Hair-Spray scaled down expectations for 2014 already, calling it impossible to win, as you quoted. This is so far removed from common sense that it's stunning. What are they thinking with this ****load of **** ?
And here we are, having the most talented squad since the seventees with a coach who doesn't seem to believe in those players at all. Between this, and Niersbach calling Löws' position 'alternativlos' (I don't think there's a correct translation for the dumbest term ever created by mankind), it's just becoming too much.