Yep, I see em. But I hadn't looked at it at as you seemed to be asking about the modules I linked to. I imagine the modules I linked to are to be designed to be used as part of a bigger design, which would include the filter. The filter specifications are going to change depending on the application, therefore I suspect it's usually left to the designer.
The DDS-60 looks to be a more completed, hobbyist targeted design.
It all comes down to your application.