Cool, but how is this bigger than, e.g. East Finland's lake landscape?

The title is incorrect. The articles’s text adds “artificial”:

“Part of Lusatian Lakeland – now Europe's largest artificial water landscape”

Even that’s debatable, I think, given, for example:

- https://en.wikipedia.org/wiki/Alqueva_Dam: a 250 km² artificial lake

- https://en.wikipedia.org/wiki/Volgograd_Reservoir: over 3,000km²

- https://en.wikipedia.org/wiki/IJsselmeer: 1,100km²

=> it depends on such things as whether you call a reservoir a lake, how you define “water landscape”, etc.