You’re working with types not instances… and for some reason group types have a workset. This baffles me, and is a Revit thing not a Dynamo thing. Notice that if you select a group in the project browser it has a workset parameter value. You can’t change this with normal Revit as the value is grayed out. It’s just there to look pretty and confuse people. With Dynamo you CAN change it. No idea why - it’s a head scratcher.
Anyway, to solve this problem, what you need to do is add a node or function to select the group instances of the given type. Fortunately there is a handy node in clockwork for this - GroupType.Groups. Add that in between your List.FilterByBoolMask and your Element.SetParameterValueByName node and you should get what you’re after. Give it a shot and let us know if you’re still stuck.