It is possible the LH's slot (either inbound or outbound or both) was cancelled 'proactively' as fog was anticipated at an earlier hour. Or their outbound flight was cancelled due fog in Munich.
That doesn't excuse LH's handling of course. However it is possible that LH did not have her contact numbers. When you book through a travel agent they often will not provide same to the airline.
There is no obligation that they must offer a voucher, although they are obligated to provide one. (Yet another example of the poor language of Regulation 261