I am not sure that the Sudeten-Gernans were the decisive issue. It seems to me that Zeman in particular won
Slovakia, and the mining areas in the North (the latter is not surprising for a Social Democrat), Schwarzenberg actually did quite well in some of the areas formerly settled by Sudetendeutsche. Especially the North-East (Liberec) area in the CZ-D-PL Triangle stands out as a non-large city Schwarzenerg stronghold.
A color-scaled map of margins might be helpful to access regional effects in more detail.
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Slovakia?! You must mean Moravia, where the left-wing parties do very well in general (except Brno). The Sudetenland is more mixed, with right-wingers strong in the north and south and left-wingers in the west. This time though, the region was very strong for Zeman, so Schwarzenberg's remarks must have had some effect. The exception is the area around Liberec (incidentally it's the fifth largest city in Czechia, so I wouldn't call it small), which is usually a right-wing stronghold, but it also had some of the the highest percentages for Schwarzenberg's party at the last parliamentary election. This area is apparently one of the few with a remaining German minority, so maybe that's why they're more receptive to him.