Yea, noone really knows where it was - or what was the exact extent od Samo's empire. The reasoning about being it south on Danube is that the core of Samo's empire was somewehere around current South Moravia and the Franks were comming most likely from the south, the best way available to them along the river, where a number of old Roman camps still existed, along with - most likely - some roads. Nothing like that was in Bohemia, just an unpopulated forrest and swamps and basically a lovely but wild countryside